Explicitly install gettext on job workers

Some providers include gettext preinstalled by default on their base
images, but others do not. Make this consistent by explicitly
installing it when not already present. Rackspace seems to have
recently ceased including it on their Ubuntu Precise images, which
precipitated this patch.

Change-Id: Idb86c805e2fc56d885081876642bd9d3dbf58434
Merge "Explicitly install gettext on job workers"