Set default sql server for mysql

Set default sql server to be tools-db, which is typically used
for user databases. The override.my.cnf is placed in mysql's
conf.d directory instead of changing the ubuntu-installed my.cnf.

MySQL parses config files in the following order [1]:
  /etc/my.cnf
    -> loads /etc/mysql.d/*.cnf
  $MYSQL_HOME/my.cnf (not used)
  --defaults-extra-file=...
  ~/.my.cnf

So this will only override settings in /etc/my.cnf, not in any
user-configured file.

[1] https://dev.mysql.com/doc/refman/5.0/en/option-files.html

Change-Id: Ifab2f3af3f4b063853345a09f2ecc3d28a82278b
Merge "Set default sql server for mysql" into production