Handle focal's insistence we don't use root in launch-node.py

It seems newer focal images force you to use the ubuntu user account. We
already have code that attempts to fallback to ubuntu, but we were not
properly catching the error when root fails which caused the whole
script to fail.

Address this by catching the exception, logging a message, then
continuing to the next possible option. If no possible options are found
we raise an exception already which handles the worst case situation.

Change-Id: Ie6013763daff01063840abce193050b33120a7a2

launch/launch-node.py

@@ -34,6 +34,8 @@ import utils
 import openstack
 import paramiko
 
+from sshclient import SSHException
+
 SCRIPT_DIR = os.path.dirname(sys.argv[0])
 
 try:
@@ -100,9 +102,14 @@ def bootstrap_server(server, key, name, volume_device, keep,
     ssh_kwargs = dict(pkey=key)
 
     print("--- Running initial configuration on host %s ---" % ip)
+    ssh_client = None
     for username in ['root', 'ubuntu', 'centos', 'admin']:
-        ssh_client = utils.ssh_connect(ip, username, ssh_kwargs,
-                                       timeout=timeout)
+        try:
+            ssh_client = utils.ssh_connect(ip, username, ssh_kwargs,
+                                           timeout=timeout)
+        except SSHException:
+            print("Username: " + username + " failed to ssh. "
+                  "Trying next option.")
         if ssh_client:
             break