Add the ability to read the output and if interactive shells are disabled on Gerrit

[jpopadak]

The SshConnection successfully opens a connection to Gerrit, executes the command, and then on its first readLine() call, the stream returns null. The GerritConnection object then keeps connecting and disconnecting every second without actually showing any errors to the outside world (minus the connect and disconnect events). It would be nice to receive an error message when some servers return the following output:

  ssh -p 29418 sshusername@hostname

    ****    Welcome to Gerrit Code Review    ****

    Hi John Doe, you have successfully connected over SSH.

    Unfortunately, interactive shells are disabled.
    To clone a hosted Git repository, use:

    git clone ssh://sshusername@hostname:29418/REPOSITORY_NAME.git

  Connection to hostname closed.

[dpursehouse]

if interactive shells are disabled on Gerrit

Interactive shell is always disabled on Gerrit. There's no 'if' about it.