Currently each charm defines in the code all the
mandatory relations. Instead populate the mandatory
relations from chamrcraft.yaml requires relations
with optional as False.
Charm can define mandatory relations that are not
derived from requires relations and they will be
appended to the list of mandatory_relations.
Barbican and ovn-central charms follows this pattern.
Change-Id: Iff45fca33dc954593ded52b97e905431b6a7bb53
e07819a9d9