[#65625634] Add config validator

Adding the config validator to core. No change.

Corresponding PRs on vcloud-tools and vcloud-edge_gateway FOR REFERENCE ONLY:
https://github.com/alphagov/vcloud-tools/pull/166
https://github.com/alphagov/vcloud-edge_gateway/pull/27
Please do not merge those PRs

This PR is the first step in moving the config loader to core: https://www.pivotaltracker.com/story/show/65625634