Add extends to buildout cache

[Gomez]

All our server use the extends cache, as it makes them more independent from third party downtimes.

We use a task like this:

- name: Copy the buildout default cfg template
  template:
    src: "buildout_default_cfg.j2"
    dest: '$home/.buildout/default.cfg'

With this template:

[buildout]
eggs-directory = $home/.buildout/eggs
download-cache = $home/.buildout/downloads
extends-cache  = $home/.buildout/extends

Should i prepare a PR for this?

[MrTango]

I'm not sure with the default.cfg but in general we should have this at least as option in the buildout config. I would prefer to have it directly in every buildout.cfg.

[smcmahon]

We've currently got:

eggs-directory=../buildout-cache/eggs
download-cache=../buildout-cache/downloads

Would it make sense to extend that to add:

extends-cache=../buildout-cache/downloads