Add support for multiple helm values files

[GeertJohan]

This PR adds support for multiple helm values files in the skaffold config.

The helm deploy config the field valuesFilePath has been renamed to valuesFiles and now accepts a list of strings, instead of a string.
All valuesFiles are added to helm deploy using the -f option.

This PR was based on the changes and discussion in #677.

[GeertJohan]

Closes #672

[nkubala]

Hey @GeertJohan, thanks for keeping up with this one. Unfortunately this won't work; since we already rolled out the v1alpha3 config change, anybody who's already used skaffold fix to update their configs will be broken by this. You'll need to make a v1alpha4 config and a new transform function instead of updating the existing one. The rest of the change looks fine though

[r2d4]

@nkubala we haven't actually shipped v1alpha3 yet, so this might be fine

[nkubala]

Oh wow my bad, somehow I thought we got that in for the release last week...I should have looked 😨 @GeertJohan disregard my last comment

[GeertJohan]

Alright, is there anything else needed or can this be merged now? How to make kokoro happy?

[GeertJohan]

Thanks @dgageot 🎉