Set up the devstack source directories
 
 Ensure that the base directory exists, and then move the source repos
 into it.
 
 **Role Variables**
 
 .. zuul:rolevar:: devstack_base_dir
    :default: /opt/stack
 
    The devstack base directory.

  .. zuul:rolevar:: devstack_sources_branch

     :default: None
 
     The target branch to be setup (where available).