Certificate addition support to mon and ro charms

This commit adds the `certificates` config in both RO and MON charms.

Changes:
 - Add `certificates` config option in mon and ro charms
 - Use charmed-osm/ops-lib-charmed-osm library in all charms

Change-Id: I14e3cf7ad2c846c4a2af57f317d4151119ce5fbb
Signed-off-by: David Garcia <david.garcia@canonical.com>