Build Variant CLI Option

Add an option to the `rez build` command to support building single variants.

This reimplements a useful feature that has been lost during the rez 2.0 effort.

The `-v` flag is used to specify the variant index (0 based) to match the behaviour with Rez 1.0.

In this implementation however it is possible to specify multiple variants to be built, for example:

    rez build -v 0 2 -- -- 

This branch was taken from the 2.0 at the beginning of the week as it wasn't clear what the state of the other branches was.  I don't think it will introduce conflicts, but might require modifications with respect to the change from `settings` to `config`.