keystone needs to notify services when endpoint changes

[Edward Hope-Morley]

It is currently all to easy to do the foolowing:

* deploy multiple units of a service e.g. ceilometer + hacluster
* deploy keystone
* add relation between keystone and ceilometer

If the service-identity relation fires prior to ceilometers vip being configured, the endpoint will be set to the ip address of the last ceilometer unit to join. Once this is set, ceilometer services will start/restart and may perform client operations that use the service catalog from keystone containing this ceilometer endpoint which has been set prior to the vip being set as the endpoint in a subsequent keystone relation-set once the vip is up.

The problem here is that subsequent calls to keystone with new endpoint addresses do not result in a refire of identity-service-relation-changed since keystone applies the same values as the previous run.

We need an unintrusive means to allow keystone to notify relations (and only ones that care to avoid noise) that the endpoint has been set to a new value.