Citrus renamed: Pulp Nodes

Summary of changes:

* General renaming of citrus -> nodes.  *Nodes* is the concept or functionality and *pulp_node* is the python package.
* Refactor code base into python packages.
* Convert plugins to entry points.
* Search *upstream* and *remote* replace with: *parent*
* Search *local* replace with: *child*
* Search *pulp_citrus* replace with: *pulp_node*

Git tree summary:

 nodes/
 ├── child
 │   ├── etc
 │   │   └── pulp
 │   │       └── agent
 │   │           └── conf.d
 │   ├── pulp_node
 │   │   ├── handlers
 │   │   └── importers
 │   │       ├── http
 │   │       └── types
 ├── common
 │   ├── pulp_node
 ├── parent
 │   ├── etc
 │   │   └── httpd
 │   │       └── conf.d
 │   ├── pulp_node
 │   │   └── distributors
 │   │       └── http

Packages (RPM):

* pulp-nodes-common
* pulp-nodes-parent
* pulp-nodes-child

Note:  Large scale file moving and renaming in addition to search and replaces make this PR look worse than it really is :)