Create a single MySQL instance when multiple keyspaces are defined

[frouioui]

Fixes a bug where multiple MySQL processes where created when multiple keyspaces are defined, instead of creating a single process.

This led to issues like this:

Unknown database 'customer' (errno 1049) (sqlstate 42000) during query: use customer

When running:

./go/test/endtoend/vtgate/vitess_tester/two_sharded_keyspaces/vschema.json ./go/test/endtoend/vtgate/vitess_tester/two_sharded_keyspaces/queries.test