Remove the reference to non existing exception by linuxbridgeplugin.

Remove the reference to non existing exception by linuxbridgeplugin.

Bug #1006221

According to plugin api of QuantumPluginBase, unplug_interface
should only raise exception.NetworkNotFound and exception.PortNotFound.
To unplug a non-attached port should not raise Exception.
After this modification, to unplug an non-attached port will have no impact.
In addition, I remove the 'network = db.network_get(net_id)' since
 'db.validate_port_ownership(tenant_id, net_id, port_id)' statement has taken
care of the check.

patch 2: split test case to test it exclusively compared to patch 1.
patch 3: remove added test statement in previous test case

Change-Id: I437c3f13fa7a81aeabcdfca7ba03e94a0a7aa32b