Do relation consistency sweep on leader change

The current charm design is to perform a sweep of all units
related on the identity-service interface to ensure that
they have all the correct setting values applied. If the
leader unit is deleted and a new one is elected this will
not happen until some event e.g. config-changed occurs. This
can result in remote units malfunctioning since they think they
are not configured. We resolve this by always doing a sweep when
the leader-elected hook fires.

Also fixes infinite loop edge case when ssl-cert-master switches
as a result of leader switch.

Change-Id: Icd68cc70d81d7d518c918e831056f686dbc7db1e
Closes-Bug: 1721269