Include schema validation and other stack validation as part of the index.json generator

[elsony]

The index.json generator is a one-stop-shop for preparing the registry index based on the content of the stacks/devfiles being added to the devfile registry. During the index generation, we need to do some basic validation, e.g. schema validation.

This item is to include:

• Schema validation to make sure all devfiles in the registry are conforming to the spec
• Produce error messages for schema validation
• Provide an option to ignore validation failure and still add the entry to the index

Related issue:
This issue is part of #82