Run pecan ceilometer tests on trusty for mongodb

The pecan ceilometer tests had been running on centos6 for mongodb. Move
them to trusty so that they can run again post trustying without a job
rename. This should prep the jobs for the next couple years as well.

Change-Id: I8ad3d4266a2a4774a3aeca78f007a54f78e8b3ec
Merge "Run pecan ceilometer tests on trusty for mongodb"