Allow to read MySQL credentials from my.cnf

Summary:
Use standard .my.cnf MySQL defaults file to read the
MySQL connection parameters.

Ref T1309

Test Plan: Tested on TC2 with .tcl sqlconnect sql7 /tmp/my.cnf

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1309

Differential Revision: https://devcentral.nasqueron.org/D1237