Add Aodh evaluation_interval

Alarm service has been moved to Aodh for a long time [1].
Therefore, we should define evaluation_interval in
aodh.conf rather than ceilometer.conf. The interval value
should be configurable as well because we can use a
custom polling config now [2]

[1] https://review.openstack.org/#/c/200593/
[2] https://review.openstack.org/#/c/572013/

Change-Id: I7adeff2dff5d6d6ae4c621e84857347995e9203a