Implement Uninstall

[vdice]

Implement uninstall functionality via uninstall.go

[jeremyrickard]

I think this has been fixed already by 237b4f5.

[vdice]

@jeremyrickard I'm not seeing an uninstall implementation in that linked commit... https://github.com/deislabs/porter-azure/blob/master/pkg/azure/uninstall.go still shows just return nil.

[jeremyrickard]

@vdice I commented on the wrrrrrrrrronnnnnnng issue :)

[jeremyrickard]

I am working on a PR for this though! I had both issues open, hence my bad commenting :)