Validate passed NTP servers and reject bad ones

Don't blindly set NTP servers. Run them through a validator first
and drop everything that doesn't match.

Golf less.

Change-Id: Ifb135977b4359f5fe0161215cec2cdd1b22244b3
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=913002
Signed-off-by: Ryan Barry <rbarry@redhat.com>