Modify buildfile to allow msgfmt to run for specific locales.

Since some of our tests require translations (specifically German), this
change will allow you to build the minimum number of translations.
Simply set 'nopo' to a comma delimited list of locales or place the
following in your ~/.candlepinrc

nopo='de_DE'
export nopo
Merge pull request #450 from candlepin/awood/specific_msgfmt

Modify buildfile to allow msgfmt to run for specific locales.