Config gopath overrides

adds two new shell variables which are respected

KUBE_EXTRA_GOPATH and KUBE_NO_GODEPS

The first will be added to the final GOPATH.  The later will cause the Godeps directory to not be added to the final GOPATH.  Used in conjunction these can be used to replace the Godeps with something outside of the tree.
Can you please update this content?

s/Set/Append/

And indicate it's done conditionally.