Create a new --no-voting option for masterfailover

This allows failing over in certain corner cases, such as a 2 node
cluster with one node down. The man page is also updated to document
this dangerous option and how to recover from this situation.

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