Add ops.scenario tests. This allows each charm class to be easily
tested with different permutations of missing/incomplete/complete
relations.
This is a starting point for using ops.scenario, additional
tests should include: examining rendered files, peer relation,
test secrets events etc
Change-Id: I8ebdad250d7cb169c3c0d72858e0582000d98b6e
0fe5f7dfb4