We've changed the config to be more consistent about "copies" and
"artifacts". This means that "copies" no longer accepts a string, but a
slice of strings. We should bump the config version and blubber version
to make it clear that we're not longer backwards compatible.

Change-Id: Ib68f623cbdcdf0aed74f4fdaec062835748e8eb9

Uses the github.com/ghodss/yaml library to convert YAML to JSON before
unmarshaling for the purposes of supporting YAML and JSON input while
converting to only support JSON internally.

Bug: T207694
Change-Id: I00668014907e9ea54917f5d5067cac08d0668053

Change-Id: I91cac046976cf88d513487e27e1b52bf77a6f974

Summary:
Introduced a `version` config field that must be specified and match
`config.CurrentVersion`.

Changed `config.ReadConfig` to use `yaml.UnmarshalStrict` to ensure that
errors are surfaced when unknown/bad fields are present in the given
YAML config. A smaller `config.VersionConfig` is now unmarshaled first
to prevalidate the new `version` field before the entire config is
parsed.

Fixes T191460

Test Plan:
Run `go test ./...`. Run `blubber` against some configuration containing
invalid fields and ensure that it surfaces a YAML error.

Reviewers: thcipriani, demon, hashar, mmodell, mobrovac, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T191460

Differential Revision: https://phabricator.wikimedia.org/D1021