Use checkPassword API

Part of https://github.com/mozilla/fxa-auth-db-server/issues/88.

Testing depends on : https://github.com/mozilla/fxa-auth-db-mem/pull/9 and https://github.com/mozilla/fxa-auth-db-server/pull/108

Launch with MySQL db depends on : https://github.com/mozilla/fxa-auth-db-server/pull/103
The `email` parameter here seems to be unused, is it necessary?