Rename masterfailover to master-failover

Most (all?) of our commands use dash-separator: replace-disks,
verify-disks, add-tags, etc. “gnt-cluster masterfailover” is an old
exception to this rule.

The patch replaces it with master-failover, add a compatiblity alias,
and updates the documentation for this change.

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