Add environment variable validation to validate-metadata.js

[SensibleWood]

User Story

As a developer of the Tooling project I want to ensure that environment variables are validated before the rest of the build executes

Detailed Requirement

Currently when a build kicks off the first step is to validate that environment variables are present, which is executed by validate-metadata.js. Currently any validation of the values is performed downstream.

To make processing more efficient this should be moved upstream so that initialisation and type checking can be performed before any further steps.