execute_ssh_command: handle 'Timeout opening channel'

This resolves recent systest/htx_hardbootme_test.robot failures that
look like this:

except_type:  paramiko.ssh_exception.SSHException
except_value: Timeout opening channel.

Change-Id: I1d5fa8353c0e11376c37a2b65c2093ec71775014
Signed-off-by: Michael Walsh <micwalsh@us.ibm.com>
diff --git a/lib/gen_robot_ssh.py b/lib/gen_robot_ssh.py
index 9954c1b..af5e375 100755
--- a/lib/gen_robot_ssh.py
+++ b/lib/gen_robot_ssh.py
@@ -302,6 +302,9 @@
                 and re.match(r"SSH session not active", str(except_value))) or\
                (except_type is socket.error
                 and re.match(r"\[Errno 104\] Connection reset by peer",
+                             str(except_value))) or\
+               (except_type is paramiko.ssh_exception.SSHException
+                and re.match(r"Timeout opening channel\.",
                              str(except_value))):
                 # Close and re-open a connection.
                 # Note: close_connection() doesn't appear to get rid of the