Cascading template system for packages and configs

[certik]

Reported by @aterrel.

Right now one of the major pain point for me using hashdist is all the
repeat in the packages. For example there is a lot of repeats in the
two files:

https://github.com/hashdist/python-hpcmp2/blob/master/package.yml.darwin
https://github.com/hashdist/python-hpcmp2/blob/master/package.yml.linux

not to mention all the fooConfig/foo.<arch>

On a call with Chris today I made the point that we should be instead
make a cascading template system for these packages and configs. My
view is that we should do something more like:

package.yml.base
...

package.yml.<arch>
  {% include package.yml %}
  <other arch dependent recipes>