Use leader set for ceilometer-upgrade checks

The previous attempt at addressing Bug#1811108 was a bit naive.
It was not HA aware and it failed to handle upgrade-charm.

This change moves from kv to leader-set as only the leader needs to run
the action and then inform the other nodes.

Note: on upgrade-charm we must assume the deployment has run
ceilometer-upgrade as there is no mechanism to determine this
independently. If we do not do this, the charm is in blocked state after
charm-upgrade. A warning is logged that this assumption has occurred.

Partial-Bug: #1811108
Change-Id: Idcc26df53542e78f0671942c99edfcbf61eccf6c