Refactor deploy/activate script

[monfresh]

Why: If the deploy/activate script is a shell script, it requires
calling Bundler.setup so that the files that the script depends on
are loaded. The problem is that the Bundler.setup call was preventing
us from running parallel builds in Circle CI.

I removed the Bundler.setup call in #2060 to see if that would allow
parallel builds, which it did, but I forgot to check if this would break
build deployment, and it did.

In order to allow the script to run during deployment and also allow
parallel builds to work, I defined a shell script that will fetch the
config from S3 via a rake task. That way, Rails and gems will already
be loaded. This also separates concerns and makes it easier to test the
S3 fetching in isolation. It also keeps things consistent with the other
scripts in the deploy directory.

Hi! Before submitting your PR for review, and/or before merging it, please
go through the following checklist:

• For DB changes, check for missing indexes, check to see if the changes
  affect other apps (such as the dashboard), make sure the DB columns in the
  various environments are properly populated, coordinate with devops, plan
  migrations in separate steps.

• For route changes, make sure GET requests don't change state or result in
  destructive behavior. GET requests should only result in information being
  read, not written.

• For encryption changes, make sure it is compatible with data that was
  encrypted with the old code.

• For secrets changes, make sure to update the S3 secrets bucket with the
  new configs in all environments.

• Do not disable Rubocop or Reek offenses unless you are absolutely sure
  they are false positives. If you're not sure how to fix the offense, please
  ask a teammate.

• When reading data, write tests for nil values, empty strings,
  and invalid formats.

• When calling redirect_to in a controller, use _url, not _path.

• When adding user data to the session, use the user_session helper
  instead of the session helper so the data does not persist beyond the user's
  session.

• When adding a new controller that requires the user to be fully
  authenticated, make sure to add before_action :confirm_two_factor_authenticated.

[mark-from-usds]

• bundle exec rake app:activate
  rake aborted!
  TypeError: no implicit conversion of nil into String
  /srv/idp/releases/chef/lib/saml_idp_constants.rb:9:in <module:Constants>' /srv/idp/releases/chef/lib/saml_idp_constants.rb:4:in module:Idp'
  /srv/idp/releases/chef/lib/saml_idp_constants.rb:3:in <module:Saml>' /srv/idp/releases/chef/lib/saml_idp_constants.rb:2:in <top (required)>'
  [skipping a few]
  /srv/idp/shared/bundle/ruby/2.3.0/gems/railties-5.1.5/lib/rails/application.rb:445:in block in run_tasks_blocks' /srv/idp/shared/bundle/ruby/2.3.0/gems/rake-12.3.1/exe/rake:27:in <top (required)>'
  /opt/ruby_build/builds/2.3.6/bin/bundle:22:in load' /opt/ruby_build/builds/2.3.6/bin/bundle:22:in '
  Tasks: TOP => app:activate => environment
  (See full trace by running task with --trace)
  ---- End output of ./deploy/activate ----
  Ran ./deploy/activate returned 1

[monfresh]

Fudge. It's the vicious cycle of needing the config to be present before loading the app. Back to the drawing board.

[monfresh]

@mark-from-usds Can you try again? I forced pushed a change.

[monfresh]

I just recycled the markryan env with the latest changes. Fingers crossed.

[mark-from-usds]

---- Begin output of ./deploy/activate ----
STDOUT: deploy/activate starting
HOME: 
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu)
/opt/ruby_build/builds/2.3.6/bin/bundle
`/` is not writable.
Bundler will use `/tmp/bundler/home/unknown' as your home directory temporarily.
bundler: failed to load command: bin/activate (bin/activate)
STDERR: + id
+ which bundle
+ bundle exec bin/activate
NameError: uninitialized constant Deploy::Activate::Rails
  /srv/idp/releases/chef/lib/deploy/activate.rb:38:in `env_yaml_path'
  /srv/idp/releases/chef/lib/deploy/activate.rb:17:in `run'
  bin/activate:5:in `<top (required)>'
---- End output of ./deploy/activate ----
Ran ./deploy/activate returned 1
[2018-03-29T13:57:15+00:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)

[monfresh]

Still not working but I know what the issue is. Hopefully the third time will be the charm.

[monfresh]

Yay! Success! Can you approve the PR please?