SSH connection related cleanups

Catching only the SSHException the AuthenticationException is
 a subclass of the SSHException in the ssh.py.

test_connection_auth method changed to exception raiser method, in order
to avid unwanted catch-and-raise-new-exception code from the
RemoteClient.

Use similar ssh connectivity check with the test_network_basic_ops,
as with all other test cases, so using the implicit
connection validation of the RemoteClient.

Improve ssh connection logging by logging the reason of the connection
failure.

Change-Id: Ia2599f7f2c2fdc6fcbf7ad3337d82adcc50e4d16
Merge "SSH connection related cleanups"