support environment variables as part of option values

[Florian Friesdorf]

I implemented support for environment variables as option values.

One patch is for enabling ${env:<variable>} syntax, wherever ${<section>:<option>} is supported. The value of an environment variable may contain a reference to a section:option. (0001-...)

The other patch enables usage of ${env:<variable>} for extends. (0002-...)

Tests are included, patches apply cleanly to 1.4.3 and trunk@110428

[Florian Friesdorf (flo-chaoflow)]

aclark pointed me to gocept.recipe.env which might be a cleaner approach, however, without support for extends. On the other hand, the extends support is unrelated to the support everywhere else.

[Marc Abramowitz (msabramo)]

I love this idea. This would make it easy to share my ~/.buildout/default.cfg file between systems.

[Alexandre Fayolle - camptocamp (alexandre-fayolle-c2c)]

The problem with the gocept recipe is that I cannot use it to set things in the [buildout] section as this causes:

    Error: Referenced option does not exist: env HOME

I'm trying to setup a the shared egg directory in a portable way via .buildout/default.cfg