Locking related fixes for networks

Use GetNetwork() only when having already aquired the lock,
i.e. in CheckPrereq().

In LUNetworkConnect/Disconnect do not include Network info in Hooks
environment, so that network locking can be avoided if conflicts
are not checked.

Invoke _CheckNodeGroupInstances() only in case of conflicts check,
because otherwise owned instances are None, which results to an error.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>