Add opcode for running commands remotely

The opcode doesn't pay attention to the build-time flag to enable or
disable restricted commands. In a cluster different nodes could have
different settings.

Node locks are acquired in shared mode by default, but the use of an
exclusive lock can be forced using the “use_locking” parameter.

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