Update rax to sync with rax_clb_nodes

Both modules dealing with rax resources should use the same arguments
and environmental variables. The author of rax_clb_nodes did some things
better so sync rax with that effort.
use home path expansion for virtualenv param in django_manage module
add a way for callback to disable itself

The idea is that some plugin would not be called in some
specific case, and the callback should decide by itself.

Having a way to globally disable it is much cleaner than
disabling every method one by one on the plugin side.

My use case is for fedora-infrastructure that cannot be run
from git checkout since it try to connect to the message bus,
but another case would be to bootstrap infrastructure, or to
run the code on a test servers without having all the callback
infrastructure setup.