Promote explict slave as a master with sentinel command

1] Spec
 -> Promote explict slave as a master.
    1] it should be a slave of current master.
    2] it should not be S_DOWN, O_DOWN, DISCONNECT status
    3] it should not be DEMOTED status.

2] Why does it need?
 sentinel supports failover using "sentinel failover [master-name]" command.
 but we can't choose specific slave as a new master. sometimes we might need to promote
 specific slave as a new master because of administration. 

3] Usage
```c
 sentinel failover [master_name] [ip] [port]
```