Adding ability to specify helm chart repo using environment variables

What does this MR do?

As part of AutoDevops, this adds the ability to specify a name for the helm repo, and a username and password. This allows a helm chart to import other helm charts from a private helm repository.

If the new variables are not specified, behaviour mirrors current behaviour.

This partially fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/49520 - I believe that in combination with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26838 everything in https://gitlab.com/gitlab-org/gitlab-ce/issues/49520 will be covered.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team