added get_variant_from_uri functionality

[nerdvegas]

This is a first step towards #680, which I'm starting to work on now (yay!). The intent here is that you should be able to munge commands together to do useful things fairly easily.

For example, to warm the package cache, you might do this:

> ]$ rez-context --resolve --show-uris | xargs rez-package-cache --add-variant

In a similar vein, perhaps you want to create a local package repo for a context, so you can create a new context using only these variants, and then copy the whole thing to a server (more work is required to make this fully standalone, but I'm keen to see this feature, as is our engineering department, who hate nfs mounts :) ):

> ]$ rez-context --resolve --show-uris | xargs -i rez-cp --variant-uris {} --dest-path /mylocalrepo