Fix exception message in Networks API extension

Raise exception indicating VLAN is disabled when
trying to associate a network w/ a tenant in cases where
VLAN is not the network manager
Added unit test case
Fixes bug 1075790

Change-Id: I3dcac09fbf06d9f64b18cc52d0b069dc9c0a7370
Merge "Fix exception message in Networks API extension"
Merge remote-tracking branch 'm_src_repo/master' into build-temp