Let method _get_ssh_connection handle timeout error.

This exception is raised when the server is not reachable,
which is exactly what is need to be handled by ssh_timeout.

Change-Id: I3e4f57531a348c3619d92807e0e762b3d0478220
Closes-Bug: 1588783
diff --git a/tempest/lib/common/ssh.py b/tempest/lib/common/ssh.py
index a831dbd..3c54cc5 100644
--- a/tempest/lib/common/ssh.py
+++ b/tempest/lib/common/ssh.py
@@ -77,7 +77,7 @@
                          self.username, self.host)
                 return ssh
             except (EOFError,
-                    socket.error,
+                    socket.error, socket.timeout,
                     paramiko.SSHException) as e:
                 if self._is_timed_out(_start_time):
                     LOG.exception("Failed to establish authenticated ssh"