Set CEILOMETER_PIPELINE_INTERVAL to 15

New incomming tempest tests around will check the notification are correctly
collected by ceilometer. But by default the notification are processed every
600 seconds. This is too high for the gate jobs. This patch reduce this
interval to 15 seconds to allow ceilometer tempest test to succeed in
less than 15 seconds instead of 10 minutes.

Change-Id: I4cb5d0eeb03b5b2868664dcb7546d6994217dc03