Do not add obsolete translation entries

msgmerge keeps obsolete translation entries but we should filter them
out. Do the filtering with msgattrib.

Note this is only needed for log files, the other files are handled
correctly since update_catalog has an option for this.

This fixes the issue seen in glance:
https://review.openstack.org/#/c/105069/

Change-Id: I956e4af59d7bbd6e90583bdb9412c9bf85b87019
Merge "Do not add obsolete translation entries"