Request: Allow to define multiple local/release folders

This includes the details for the request.
https://groups.google.com/forum/#!topic/rez-config/nazmdCCg1XM

In a nutshell, in the config:

- local_packages_path (Str) becomes local_packages_path**s** (StrList)
- release_packages_path (Str) becomes release_packages_path**s** (StrList)
- The default selected path is set via packages_index (Int)

The benefit is that now different paths might act as local/release and will be correctly discovered.

To release/build to a different local/release folder you just need to override the packages_index in the package.py:

~~~python
config = {
"packages_index": 1,
}
~~~

For the sake of clean implementation it does break config compatibility (to some little degree).
For example:
~~~python
local_packages_path = "~/packages"
release_packages_path = "~/.rez/packages/int"
~~~
Becomes:
~~~python
local_packages_paths = ["~/packages"]
release_packages_paths = ["~/.rez/packages/int"]
packages_index = 0
~~~

I will make a pull request for this feature soon.

Start with the configuration keys described in the issue and the package.py packages_index example. Trace how local and release package paths are discovered, then confirm that list-valued paths, index selection, and the documented configuration migration work as described.

Written by the indexing model from the issue text.