Title: Passwordless MySQL Health check

Description:
For feature parity with HAProxy, Envoy should have support for basic mysql health check. It involves sending a login request for a passwordless user, followed by a quit request.

Relevant Links:

• HAProxy documentation: http://cbonte.github.io/haproxy-dconv/1.5/configuration.html#4-option%20mysql-check
• User group discussion: https://groups.google.com/forum/#!searchin/envoy-users/feng$20pan%7Csort:date/envoy-users/SVjtIHa--Lo/YmdpaHvNAQAJ