Add the ability to load a remote environment values file

[kevinjqiu]

This PR enables the user to specify a remote path for an environment values file, e.g.,

environments:
  cluster-azure-us-west:
    values:
      - git::https://git.company.org/helmfiles/global/azure.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/us-west.yaml?ref=master
  cluster-gcp-europe-west:
    values:
      - git::https://git.company.org/helmfiles/global/gcp.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/europe-west.yaml?ref=master

releases:
  - ...

This is particularly useful when you co-locate helmfiles within your project repo but want to reuse the definitions in a global repo.

Please let me know what y'all think. Thanks.

[mumoshu]

@kevinjqiu Hey! Thanks a lot for your contribution. LGTM.

Just wanted to ask if you have any idea how we could add some tests for this. Mocking out Remote for unit testing, create a real github repo for E2E testing, etc.?

[kevinjqiu]

@kevinjqiu Hey! Thanks a lot for your contribution. LGTM.

Just wanted to ask if you have any idea how we could add some tests for this. Mocking out Remote for unit testing, create a real github repo for E2E testing, etc.?

Thanks @mumoshu

wrt testing, what you said makes sense. E2E testing should be emphasized IMO.

[lodotek]

has this been tested against private gitlab repos?