[WIP] Working directories for tasks

This PR introduces a 'working_directory' config in /etc/pulp/server.conf in the [server] section. This is the path where each pulp worker can expect to have a directory with it's name.  Inside that worker's directory, each task will be able to create working directory for that task.   

Take note of the changes in server/pulp/server/managers/repo/cud.py.  I decided to remove this code because we no longer expect to have to clean up repository working directories outside of tasks.  Let me know if I am misunderstanding something.  

Still a work in progress because the spec file needs to be modified to create /var/cache/pulp and then tested with SELinux.
this is a 3rd-part library, so it needs to be in its own import group below ``unittest`` and above ``pulp.common``.