Adding qpid support to ceilometer

Moving all qpid parameters under default["openstack"]["metering"]
["mq"]["qpid"] so that other qpid options can be added under it.
Rabbit parameters will be brought under "mq" eventually. MQ is
selected through default["openstack"]["metering"]["mq"]["service_type"].
Default is "rabbitmq".

Implements: blueprint qpid-activemq-support
Change-Id: I4dd153a631063ee7c518e2d49fe01aa39a6f261d
Relax the dependency on openstack-identity to the 7.x series

openstack-identity is now on 7.2.0, which doesn't get picked up because of the
strict dependency of the 7.0.x series. CHANGELOG cleanup.

Change-Id: I790fbf9c9db83d73eca1779798b9356089e545db