commands: Add ssh protocol-friendliness to rocks run host

The nodeup() method currently opens a socket, receives an identification string and closes the socket implicitly when it probes for whether the host is up.

If an identification string is not likewise sent from the client before closing the socket, sshd logs the following error on the node:
    "Did not receive identification string from 192.168.xxx.xxx"

Even if the identification string is sent, when the socket connection is severed after sending it, sshd logs this:
    "fatal: Read from socket failed: Connection reset by peer"

From an IDS perspective, these types of SSH errors are indicative of a scan attempt and can set off alarms. 

To prevent SSH errors when probing a node, the nodeup() method should open a socket, receive an identification string, send an identification string, receive a key exhange (kex) algorithm string, and then close the socket. I don't know if closing the socket implicitly or explicitly matters in this case. I did it explicitly out of habit.

By following the above steps of the SSH protocol, probing a node only emits an informative log:
    "Connection closed by 192.168.xxx.xxx"