Add new ceph-fs configuration test

[ethanmye-rs]

This PR adds functional tests for the charm-ceph-fs config options added in the following review:
https://review.opendev.org/c/openstack/charm-ceph-fs/+/842555

[ChrisMacNaughton]

This isn't really a functional test; it's purely a test that config was written to a file, without any verification that Ceph even cares about that config line. Broadly, the desired approach for functional testing is to verify that the things you're trying to do actually causes the desired effect; as a basic example: testing that a CephFS share can be created, written to, and read from is infinitely more valuable than testing that we wrote the config we think we did.

It's possible to get the same confidence int his change via unit-testing in the charm itself, rather than cating the ceph.conf to ensure that the expected changes appeared.

[ethanmye-rs]

This isn't really a functional test; it's purely a test that config was written to a file, without any verification that Ceph even cares about that config line.

In my experience, Ceph ignores invalid config options in the .conf file, but that is anecdotal. From our earlier discussion, I agree querying the Ceph daemon directly is a better test to ensure Ceph actually understood and accepted the config values passed.

It's possible to get the same confidence int his change via unit-testing in the charm itself, rather than cating the ceph.conf to ensure that the expected changes appeared.

I've left the unit tests as they are in the opendev repo and updated this repo with the 'functional' tests that ask Ceph for the configuration directly. Thanks for the help!

[ChrisMacNaughton]

I just have one issue with the tests now, it looks like a debug print was left.