webadmin: Add network custom properties in Setup Networks

Added a custom properties widget to the edit network dialog inside
Setup Networks in case the host is in a cluster that supports the
feature, applied validation and made sure changes are flushed to the
backend entity.

Change-Id: I1bc9f38f48fcf1bd9998228d316093db2db69782
Signed-off-by: Lior Vernia <lvernia@redhat.com>