Fix --microstack flag in charmed installer

This commit does several things:
- Geneneral fix for installing the most stable release of microstack
- Configure and set up microstack so it is ready to use: create an
osm-ext network, a router, configure the security groups etc; so it is
ready to be used by OSM
- Add xenial, bionic, and focal images to microstack
- Add microstack as VIM to OSM

Change-Id: Id15531ee94c893e2670b7ee8a89b2eb4e55cead0
Signed-off-by: David Garcia <david.garcia@canonical.com>