commands: Add ssh protocol-friendliness to rocks run host

To prevent SSH errors when probing a node, the nodeup() function now receives an identification string, sends an identification string, receives a list of available key exchange algorithms, and then closes the socket.

If the identification string is not sent before closing the socket, sshd logs the following error on the node:
    "Did not receive identification string from 192.168.xxx.xxx"

Similarly, when the socket connection is severed after sending the identification string, 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 will set off alarms. 

By following the above steps of the SSH protocol, probing a node only emits an informative log:
    "Connection closed by 192.168.xxx.xxx"