Added gettext to install_prep_packages

To verify the validity of po/pot files, there is now a dependency
on the msgfmt utility found in the gettext package.
(This was introduced by https://review.openstack.org/#/c/84239 )

The gettext package has therefore been added to the redstack
install_prep_packages function.

Change-Id: Iba6688dc3ef77ce6669253115d70a908f089ccd6
Closes-Bug: #1317652
Merge "Added gettext to install_prep_packages"