Major improvement in OSM charms

- Adapt all new operator charms to use the same pattern. They are all
using now this library that encapsulates the common logic for all
charms: https://github.com/davigar15/ops-lib-charmed-osm

. That will be
eventually moved to gitlab, when it has a PyPI repository available
- Add unit tests to all charms
- Modify installer and bundles to point to the new charms
- Improve the build.sh script for building the charms

Change-Id: I0896ceb082d1b6a76b3560c07482a4135a220a3f
Signed-off-by: David Garcia <david.garcia@canonical.com>