Refactoring RO Charm to use Operator Framework

This refactoring work includes tests.

Note 1: old charm is in ro-k8s folder.
Note 2: relation-departed is currently not tested because there is
        no function to remove a relation in the Testing Harness.
        There is currently one issue open and the Charmcraft team
        should provide feedback soon.

Change-Id: Ia97de802aec2c4e10a1d2c86ba2515d3f17f55af
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>