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

Summary:
It occurred to me while looking at code in another patch that there is
no need to have this function be public.

Test Plan: `go test ./...`

Reviewers: dduvall, #release-engineering-team

Reviewed By: dduvall, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D1012

Summary:
Implements a rough interface for validating configuration against
arbitrary policy rules. Policies are provided as YAML and passed via the
command line as file paths or remote URIs.

The format of policies is:

    enforcements:
      - path: <path>
        rule: <rule>

Where `<path>` is a YAML-ish path to a config field and `<rule>` is any
expression our config validator understands (expressions built in by the
validator library and custom tags defined in `config.validation.go`).

Example policy:

    enforcements:
      - path: variants.production.base
        rule: oneof=debian:jessie debian:stretch
      - path: variants.production.runs.as
        rule: ne=foo
      - path: variants.production.node.dependencies
        rule: isfalse

Command flag parsing was implemented in `main.go` to support the new
`--policy=uri` flag and improve existing handling of `--version` and the
usage statement.

Test Plan: Run `go test ./...`.

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

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D999