Do not require storage configuration for database migrate CLI commands

So far, to run any of the database migrate subcommands, the CLI will halt with the following error if no storage section is included in the provided configuration file:

configuration error: parsing .../config.yml: no storage configuration provided

Given that storage drivers are not required to manage database migrations, we should not require the storage section to be present (or valid) when running any of the database migrate subcommands. This MR ensures that's true.

Related to gitlab-org/charts/gitlab#2567 (comment 530494738).