SshConnection: do not try to sudo as root

If a command is executed "as_root", SshConnection always prepended
"sudo" invocation to the command, even if the user name is "root". This
causes problems on patforms that do not have "sudo".

This commit changes this behavior so that sudo is never used for root
users.

devlib/utils/ssh.py

@@ -228,6 +228,9 @@ class SshConnection(object):
 
     def _execute_and_wait_for_prompt(self, command, timeout=None, as_root=False, strip_colors=True, log=True):
         self.conn.prompt(0.1)  # clear an existing prompt if there is one.
+        if self.username == 'root':
+            # As we're already root, there is no need to use sudo.
+            as_root = False
         if as_root:
             command = "sudo -- sh -c '{}'".format(escape_single_quotes(command))
             if log: