webadmin: Added required column to cluster table in Add Network

Added said column, which enables the user to mark whether an internal
network should be required or not (an external network must not be
required).

Bug-Url: https://bugzilla.redhat.com/924762

Change-Id: Id6de5dc8583d15ab5ec82df6aac07d5954418056
Signed-off-by: Lior Vernia <lvernia@redhat.com>