sshDriver: Correct handling when SSH authentication fails

When the SSH authentication failed, the SSHDriver was left in
an ununsable and inconsistent state.

Define the variables as follows:
 connected_ : when != 0, the connection is open and usable for a user
 session_   : when != NULL, it must be disconnected in disconnectSSH()
 sock_      : when != -1, the socket must be closed in disconnectSSH()

Add initialization of member variables in the constructor
(especially the is_tcp_not_ssh_ was sometimes != 0)