TelnetConnection: allow username and/or password to be optional

If username is set to None, no '-l' option is appended to the telnet command.
If password is set to None, devlib does not wait for a password prompt when
connecting.

devlib/utils/ssh.py

@@ -83,9 +83,17 @@ class TelnetConnection(pxssh.pxssh):
 
     def login(self, server, username, password='', login_timeout=10,
               auto_prompt_reset=True, sync_multiplier=1, port=23):
-        cmd = 'telnet -l {} {} {}'.format(username, server, port)
+        args = ['telnet']
+        if username is not None:
+            args += ['-l', username]
+        args += [server, str(port)]
+        cmd = ' '.join(args)
 
         spawn._spawn(self, cmd)  # pylint: disable=protected-access
+
+        if password is None:
+            i = self.expect([self.original_prompt, 'Login timed out'], timeout=login_timeout)
+        else:
             i = self.expect('(?i)(?:password)', timeout=login_timeout)
             if i == 0:
                 self.sendline(password)