Define Config.__contains__().

This fixes a breakage in migrate-mailhook-config, which used to use

    old.has_key(name)

but was changed in 09d0d5b922 to use

    name in old

.

Reported-by: Azat Khuzhin <a3at.mail@gmail.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>