Add a __contains__ method to Config to address pep8 issues

In 09d0d5b (Get Python files to pass pep8's tests. 2014-05-09), there were
a few changes from 'old.has_key(name)' to 'name in old'.  However, our
Config class only supported the has_key() API, resulting in an ugly
traceback when users went to run migrate-mailhook-config.  Add a
__contains__ method to allow the new call style to work.

Reported-by: Azat Khuzhin <a3at.mail@gmail.com>
Signed-off-by: Elijah Newren <newren@palantir.com>