Remove force_master support from LUOobCommand

As per discussion on the man-page[1] update, this functionality should be
removed and replaced by just give the command to run if the user insists
of power cycle/power off the master and refuse to operate.

[1] http://groups.google.com/group/ganeti-devel/browse_thread/thread/95d4879a747cc295



This also removes the --all flag from gnt-node power because it was
buggy and would have added too much logic for special cases.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>