Make http aware of authentication config.

* bzrlib/transport/ftp.py:
(FtpTransport._create_connection): Also provides port and path to
get_credentials since we know them.

* bzrlib/transport/http/_urllib2_wrappers.py:
(AbstractAuthHandler.get_password): Try the authentication config
before prompting the user.

* bzrlib/tests/test_http.py:
(TestAuth.test_no_prompt_for_password_when_using_auth_config):
Corresponding test (applies to raw http and proxy).