Rewrite charm using template-python-pytests

[Alvaro Uria]

Bug to track efforts to:
1) support unit and functional tests
2) rewrite the reactive script into modules (helpers)

[Alvaro Uria (aluria)]

Efforts also pushed at [1]. For now, lint or tests don't work, but the lib/lib_grafana/* package has been split in multiple files, and code in reactive/grafana.py has started to point to the defined classes and methods.

Work on fixing [1] branch will be delayed since this is not my primary assignment.

1. https://code.launchpad.net/~aluria/grafana-charm/+git/grafana-charm-1/+ref/bug/1822329-rewrite-01

[Alvaro Uria (aluria)]

Available stacked MPs (rewrite branch is a copy of master, for now):

[1] -> [2] -> rewrite

1. https://code.launchpad.net/~aluria/grafana-charm/+git/grafana-charm-1/+ref/bug/1822329-rewrite-01
2. https://code.launchpad.net/~aluria/grafana-charm/+git/grafana-charm-1/+ref/add-pytests

[Alvaro Uria (aluria)]

Initial phase of the rewrite has been completed and tested. Trusty support has been removed because snaps cannot work on trusty containers, and layer-snap returns an error instead of blocking the unit.

I will link another MP once it is ready, adding unit and functional tests for the different modules. Once approved, I think the "rewrite" branch could be merged into "master" to start fixing bugs directly into the main stream.

Plan is to file different bugs to reduce the usage of sqlite instructions and use the API or new features from v5 and on (ie. provisioning feature, for static datasources [pulled from relations] and dashboards [rendered from included templates]).

[Alvaro Uria (aluria)]

Multiple development cycles. Need to catch up with current master, and probably rebase these changes into an updated (in sync with master) "rewrite" branch.

[Xav Paice (xavpaice)]

At this stage, the team is focusing on using Zaza for functional tests.

Unit tests are implemented.

The charm is rewritten in a more modular way.

Given these, I'll mark this bug as fix-released.