webadmin: Reset properties when detaching network

Previously the underlying network interface entity's properties were
not properly reset when a network was detached, regarding
recently-added properties (QoS, custom properties). This is now fixed
for custom properties.

Change-Id: I37f3f1bc0cc3d5353c43d628fa3b24420728236c
Bug-Url: https://bugzilla.redhat.com/1119019
Signed-off-by: Lior Vernia <lvernia@redhat.com>