engine: allow values higher than 9 minutes for CpuOverCommitDuration

Change-Id: I1b8b4f7b4f8ef76dc9774dc1620e6bb76aaf2f65
Bug-Url: https://bugzilla.redhat.com/1073776
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>