default username/password values for mysql are causing issues with env vars

[groob]

kolide sets the default mysql username and password to kolide

I created a new mysql instance which only has a user but no password:

export KOLIDE_MYSQL_PASSWORD=""

On linux(alpine here) this value is never set, so the default is still used.

Using a flag or a config file to override works, but it's ugly.

./kolide serve --mysql_password=""

There should not be default values for mysql username and password. We should remove the defaults and update our compose files used for testing.