Make smtp aware of authentication config.

* bzrlib/smtp_connection.py:
(SMTPConnection._authenticate): Try the authentication config
before prompting the user.

* bzrlib/tests/test_smtp_connection.py:
Fix some imports.
(everybody_is_welcome): Fake auth smtp server.
(TestSMTPConnection): Now inherits from tests.TestCaseInTempDir so
that we can create an authentication.conf in a protected env.
(TestSMTPConnection.test_smtp_password_from_auth_config): New test.
(TestSMTPConnectionWithUI): New test for prompting user.