Disallow non-admin users update net's shared attribute

Currently non-admin user cannot create a network with
shared=True. But the user can create the network and then
change the shared attribute to True.

This patch will no longer allow non-admin user to update a
network's shared value to True.

Change-Id: Id596ee399c56b9882efab97a89dbf7d14c5cf7f4
Closes-Bug: 1268823
Merge "Disallow non-admin users update net's shared attribute"