Don't match commented [DEFAULT]

Class ceilometer::agent::compute fails when there are [DEFAULT] sections
commented. This patch is fixing this issue. For more info check
https://bugzilla.redhat.com/show_bug.cgi?id=1060712

Change-Id: I6d13f780f63af8798d94b49246576d1e144a4540
(cherry picked from commit a46bbc4c5179d4988f4beec0fc548c91300008a2)