Fix use of mysqladmin in mtr

Summary:
Feature: Basic Test Fixes

This changes the invocation of mysqladmin to always have the
--defaults-file option, to avoid picking up local my.cnf files.

Test Plan:
Created a ~/.my.cnf with bad creds - containing just:
  [client]
  user=superman
  password=hunter2
And ran:
mtr --force mysql_config_editor mysqladmin

Both of these tests fail without this diff, and pass with it.

Also ran the full mtr suite (with this diff included) with this bogus ~/.my.cnf present - all pass.

Reviewers: inaam-rana, darnaut, jeremycole, chip, pivanof

Reviewed By: chip, pivanof

Subscribers: chip, jtolmer

Differential Revision: https://reviews.facebook.net/D30753