Fix mysqldump generating use database

[greg-rychlewski]

Closes #539.

There are 2 things:

1. Allowing multiple prefixes for mysql requires the --databases flag, which automatically adds the use statement. So don't allow this for MySQL.
2. When dump_cmd was created the run_with_cmd was refactored to add the --databases flag. Most likely in attempt to reduce duplication/clean up the code. Putting the database specification back into the callers because they are too different to generalize here (i.e. dump_with_cmd wants to use the config database, but structure_dump wants to allow custom prefixes).

[greg-rychlewski]

i formatted one of the files accidentally =/. i will undo it

[josevalim]

@greg-rychlewski btw, feel free to format as many files as you want in separate commits. I did it for some of the Ecto stuff already. It has been too long. :) In such cases, format, refactor any code that gets too nested, and commit+push directly, no need for PRs. :) We don't have to do it all upfront either. In batches we will eventually do it.