(MODULES-1231) Fix apt::force locale issues

The current $install_check variable greps for 'Installed' or 'Candidate', which means that it will give the wrong results when a non-English locale is used. This patch ensures that the check will work properly for non-English locales.

I have done basic testing with locales fr_FR.UTF-8 and en_US.utf8.
I'm not sure how to create an acceptance test for this since it might require a re-login for locale changes to take effect.