980150 - support broker host that is different than pulp host.

https://bugzilla.redhat.com/show_bug.cgi?id=980150

Summary of changes:
- Removed macros from the pulpplugin.conf that used to synchronize the plugin's configuration with consumer.conf.  Added complexity of using the server.host when messaging.host was not specified made this necessary.
- Updated pulpplugin.py (gofer plugin):
  - Use the pulp consumer configuration instead of the plugin's configuration for pulp related properties.  This replaces the macros that used to do the same thing in pulpplugin.conf.
- Minor changes to improve test-ability; pep8 and  builtin's shadowing.  Improved docstrings and made a few methods static because I could.
- Move configuration loading to (new) pulp.client.consumer.  Described below.
- Mostly re-wrote unit test for pulpplugin.py.  Improved coverage from about 10% to 100%.  Been meaning to do this for a while.
- Added common method for loading and validating the consumer.conf.  This is intended to be shared by both the agent and the client (CLI).  I will add a story to the backlog for updating the client framework to accept a Config object instead of a list of config file paths.  Seems to make more sense to have the admin and consumer clients pass in a validated configuration instead of a list of paths.
- Reformatted the consumer.conf to match the server.conf.  When adding sections/properties that were removed from pulpplugin.conf, I notice the formatting in the file was a bit of a hodge -podge.