Allow environment vendor overrides

[slim-bean]

Look for a vendor folder in the environment which will let you jb install a dep into the environment to override what's in the main project vendor folder

[cyriltovena]

Ho wow, I needed this for one loki cluster.

[slim-bean]

I'm not sure if I'm making a mistake with the change to find the root jsonnet file by skipping up a directory. I think this is ok as I don't think it's valid for a flat structure? (unless i'm misisng a use case/requirement). That is to say, there will always be a root jsonnet.json file and then an environments folder below containing the environments with main.jsonnet?

[slim-bean]

After some discussion with @sh0rez we backed out most of the original changes to this PR and instead are just adding a check for a local vendor folder in the base dir of the environment.

You won't be able to use jb currently to add environment specific overrides but you can use wget to add anything you want overridden.

[sh0rez]

LGTM for Ed's parts

[rfratto]

Awesome!