DPE 71 Patroni resource install

[marceloneppel]

Issue

In short this PR partially address JIRA ticket DPE-71. The remaining implementation is split in three separate PRs to make the review easier.

In long:

• In order to enable automatic failover and replication through Patroni in a PostgreSQL deployment we need to change the way PostgreSQL is bootstraped/started (Patroni should be started; it'll start PostgreSQL later).
• Patroni is released as a Python package, which should be installed by the charm to later be started.

Solution

• Enable the human operator to inform a resource containing Patroni Python package when deploying.
• Installing this resource together with PostgreSQL database in the charm

Context

• Additional Patroni dependencies (like python-dev and psycopg2) will be changed to be installed as offline packages in a future PR.

Testing

• Unit tests
• Some manual testing to ensure the charm still gets to the desired state
• Update of the tox.ini file and the deploy integration test to download and add Patroni as a resource (the successful install of the resource will be better tested in the next PR, where the bootstrap process is switched to use Patroni)

Release Notes

• Add Patroni as a resource
• Add pip as a charm-python-packages part for bionic series
• Install Patroni dependencies (python-dev and psycopg2)
• Implement logic to install Patroni Python package
• Update apt update unit test matching the implementation of the test_install_pip_packages unit test

[jnsgruk]

Overall looking good, thanks Marcelo!

[jnsgruk]

Could this be some sort of fixture, or downloaded in the pytest code using urllib then cleaned up after the test?

The benefit here, is that once you have an uploaded version on chamrhub, you can fetch the resource straight from Charmhub using the API and get a bit more certainty in the test? (Perhaps a future PR?)

[marceloneppel]

Yes, it can be, Jon. Nice suggestion! I'll create a new PR to change this that and upload the resource to Charmhub to have this improved flow of testing with the resource. I created an issue to remind about it.

[MiaAltieri]

Great stuff! Amazing PR description and some super thorough commenting. Also thanks for making the PR into two chunks, this is super manageable.

Just have one concern about error handling in install, feel free to PM me if you want to chat about it 😄 Other than that, it looks great!

[paulomach]

LGTM, considering Mia's comments

[MiaAltieri]

Looks great to me! I like how you handled those cases on failure to install. Great work.

Made some suggestions to the tests but if you disagree then feel free to ignore my suggestions :)