Refactoring NBI Charm to use Operator Framework

This refactoring work includes tests.

Note 1: old charm is in nbi-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: I25b94d205d2a004946189a231b5309da1deaa8ed
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>