Fix python tests on master

Reviewed at https://reviews.apache.org/r/18248/
Add kevints CODE SIGNING KEY to KEYS.

Reviewed at https://reviews.apache.org/r/18247/
Use user.signingkey for tag-signing.

Rather than use UID require an explicitly-set key ID, following recommendations at [1].

[1] http://www.apache.org/dev/openpgp.html

Testing Done:
Tested with user.signingkey set and unset on local merged master.

Reviewed at https://reviews.apache.org/r/18245/