Updates a catalog's header to from the template

A language specific catalog's initial header is based on the template
when it is created.  Updates to the template's header were previously
not getting into the catalog during the update_catalog process.

This came from trying to update the copyright holder in the (Keystone project)[https://git.openstack.org/cgit/openstack/keystone].  I had to edit the generated .po files by hand since the headers were not being updated.