Fixes #7919/BZ1151633: fix translations of sync plan intervals.

The sync plan interval strings were being translated in the UI
and thus would fail when creating/editing a new sync plan in
another language.  This commit fixes the issue by differentiating
between interval display value and stored value and also fixes
an issue where you had to select an already populated interval
in order to create a sync plan.

http://projects.theforeman.org/issues/7919
https://bugzilla.redhat.com/show_bug.cgi?id=1151633