Add support for registryRepoURL env var to the registry deployment.

[thepetk]

/kind task

Which area/kind this issue is related to?

/area registry

Issue Description

The devfile-registry deployment should include an env var that will contain the URL of the monitored registry repository. The URL should be optional, and if not set this means that the admin of the registry doesn't want to update the registry after an update in the stacks of the github repo.

Acceptance Criteria

• The devfile-registry deployment includes an ENV variable that is optional and has the value of the registry repo that is used to generate the index.json & to fetch all the artifacts.