Fix locking in networks

Ensure that locks are held only if needed.

Add conflicts_check in OpNetworkAdd. This is needed if we want to
check whether nodes/master IPs are included in network.

Depending on conflicts_check value, we have to hold node/instance locks
during OpNetworkAdd/OpNetworkConnect or not.

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