Rename "default" recipe to "server".
Install python.
Install python virtual environment.

The devpi server will be installed into a python virtual environment in
the /opt tree.
README.md: Line wrap the "Contributing" section.
Install devpi-server package.
Create privilege separation user.
Create administrative group.
metadata.rb: Add recipe statement.
Add support for Enterprise Linux.

Enterprise Linux was already supported but I made it official by
documenting it and testing it on CentOS.
Merge branch 'feature/server-recipe' into develop