Avoid cloudwatch endpoint retrieve when disabled
Avoid cloudwatch endpoint retrieve when disabled

If openstack-api-cloudwatch service is disabled in a
deployment these lines of code will provoke a
traceback when trying to get the cloudwatch
end-point.

Change-Id: I7257f79af764cddc16423826143b0d8babfe54e7
Partial-Bug: #1715083

noironetworks/heat,openstack/heat,noironetworks/heat,openstack/heat