We want to allow operators to override Horizon's default
settings. This involves moving local_settings.d out of the read only
snap filesystem, and into $SNAP_COMMON. This is a little bit tricky.
First, we patch settings.py and local_settings.py as we're building the
snap, to include a LOCAL_PATH in $SNAP_COMMON.
Then, we add a template with the rest of our default overrides,
and write it out to $SNAP_COMMON/horizon/local_settings.d
Finally we tweak our tests so that we can give our overrides a
spin. As a bonus, this makes test_horizonglogin.py a lot easier to run
in our multipass testing scenario!
`tox -e basic` now also runs selenium tests, as well.