Ignore the generated file ceilometer.conf.sample

Since we have removed the ceilometer.conf.sample file and added
tox -e genconfig to generate it, we should no longer track it.

Change-Id: I66a03eb2672941ba5115bf0adb9fb0c944f68e4d
Closes-Bug: #1320150
Merge "Ignore the generated file ceilometer.conf.sample"