Add gnt-cluster commands to toggle the master IP

lib/client/gnt_cluster.py:
* Add activate-master-ip and deactivate-master-ip commands

man/gnt-cluster.rst:
* Document the new commands

lib/opcodes.py lib/cmdlib.py
* Add two opcodes and the LU that call the relevant RPCs

test/docs_unittest.py
* Silence an error about RAPI not implemented for the two new opcodes

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>