Add support for configuring coordination/backend_url

Add support for configuring coordination/backend_url

If 'coordination_url' argument and value is provided to the central agent
or alarm evaluator classes, they will try to set 'backend_url' in the
'[coordination]' section of the ceilometer_config. This allows group
membership handling coordinated by tooz.

ensure_resource is used to avoid duplicate resources while still allowing
the central agent and alarm evaluator to be configured separately if
on different machines.

Note that this change does not include support for enabling coordination
of the compute agent.

Change-Id: I707a8b70ad742cc9f8fdc62c196d0e75574a50b2