Support specifying custom values files

[zoidyzoidzoid]

Helm has code for this here: https://github.com/helm/helm/blob/master/cmd/helm/install.go#L358

It would probably be a better approach to try move their code for it to k8s.io/helm/pkg/chartutil, and then use that here. For now I just copied out the necessary bits, because their version supports things like custom protocols, like values files from ftp or http, which I don't think we need. Though matching what helm supports is probably a better idea.

TODO: I still need to add tests for this.

Closes https://github.com/Azure/draft/issues/818 Closes https://github.com/Azure/draft/issues/856

[msftclas]

All CLA requirements met.

[cs-andros]

Any word when, or if, this will be merged? This is a must-have feature for my particular use case.

[ofekih]

Is there any way I can help get this pull request back alive? I can contribute to the code if necessary, but like @cws-credsimple, this is very important for my use case