Allow reconfiguration of behat.yml context params.

[dkinzer]

REF CIVIC-6614 GetDKAN/dkan#2025 #332

Adds some new build tools to allow sites to override behat cotext attributes
with custom settings set in cofig.yml

See .ahoy/site/tests/test_config-behat.rb for details of how this works, but the gist of it is
you add overriding configurations in config.yml

behat:
  contexts:
    datasets:
      defaults: {}
      fields: {}
      labels: {}
      sets: {}
    services:
      request_fields_map: {}

And this gets incorporated into the various behat.yml settings by running

ahoy build config`

QA Steps

• run ahoy build test, if when tests pass... check out file in .ahoy/site/tests/test_config-behat.rb verify you understand what is going on and how this works.
• If you wish recreate those tests manually yourself and verify they work.

Required PRS.

• Add configurable arguments via Behat Contexts. dkan#2025

as per usual.