amp config - Clarify MySQL permissions

[totten]

When prompting user for administrative credentials, specify the user credentials required.

Some developers instinctively want to add a new user. Buildkit will do this automatically for each build, but they still want to create a new user for buildkit per-se. We should display instructions for grantin permissions to this user, eg

GRANT ALL ON *.* to '$user'@'localhost' IDENTIFIED BY '$pass' WITH GRANT OPTION

[TobiasLounsbury]

#12