webadmin: Network dialog- default/custom mtu as radio buttons

Substituting the 'override mtu' checkbox with two radio boxes-
'default' and 'custom'.

Bug-Url: https://bugzilla.redhat.com/1043808
Change-Id: I9f813fd292b56407c97ae69c1580061bbc0eef04
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>