Mailpass

Closes #5 (closed)

Add new routes for mailpass on /v1/mailpass and certs on /v1/certs while maintaining backward compatibility with certs on route /v1/. This new routes allows us to provide certs-like interface for obtaining mailpass (password for client authentication for our smtp server).

mailpass shares the same authentication mechanism as certs, also the same auth session format in Redis.

The only change made to the original certs mechanism is addition of action field to Redis auth session. This filed allows us to check and maintain consistency of 'actions' among distinct client requests linked to a single session.

To implement mailpass one new Redis key was added. This key, "mailpass":sn stores the password provided by CA-like box (which name will be stated) and will be read by cert-api.

The password itself is usually called secret in the code, as it is not yet decided whether it will be used as is or somehow modified.