Explain why mysql.host is written in hard

Summary:
All settings are passed through variables, except mysql.host.
Documenting the reason why this one is not.

Reviewers: dereckson, xcombelle

Subscribers: Nasqueron Docker deployment squad, xcombelle, Sandlayth, dereckson

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D615