Added a i18n module for gettext functions. (#1079440)

This keeps the gettext function definitions all in one place, and makes
it easier for other software to import blivet.i18n to get
blivet-specific translation functions.

Removed unused translation functions.

Resolves: rhbz#1079440

(cherry picked from commit bde4dcedd4f90e0504a812115e5d387f86381e7d)