Force conflicts check in LUNetworkDisconnect

Until now if one disconnects a network with --no-conflicts-check
and then remove it, there is a possibility to leave instances with NICs
referencing non-existing networks. This causes network queries,
instance removal and modification to fail.

This patch allows a network to be disconnected from a nodegroup,
only if instances residing in the nodegroup do not have NICs attached
to the network. Otherwise OpPrereqError is raised. The
--no-conflicts-check option is removed from the gnt-network disconnect
command as well.

This is a cherry-pick of commit 6bb939a2.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>