Change default age of purge_images_table to 180
Change default age of purge_images_table to 180

The `glance manage purge-images-table` is supposed to be used
only if really required due to OSSN-0075. And the advice is
not to purge any freshly deleted rows due to the possible
ID collision of still in use images. This change increases
the default of "age_in_days" to 180 from it's original 30 to
stress this out. User can still define smaller age if they
must.

Change-Id: Idb13288b72b31940a357fc9b11db2d6bcd396261

openstack/glance,openstack/glance,openstack/glance