LUInstanceQueryData: Don't acquire locks unless requested

Until now LUInstanceQueryData always acquired locks for the instance(s)
and nodes involved. In combination with long-running operations this
prevented the use of “gnt-instance info”, even with the “--static”
option. With this patch, locks are only acquired when explicitely
requested in the opcode (like all query operations).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>