Add a default variable plugin

As it stands right now, there are a number of scenarios where we would
like to be more DRY. We have multiple environments that we support and
copying and pasting variables from one environment to the next is a
recipe for disaster. Therefore provide an optional group_vars wrapper
plugin.
pep8 improvements

Fix all pep8 failures for the default_vars plugin.
Add vars_plugin declaration to ansible.cfg

With the introduction of a default_vars plugin, add its path to the
ansible configuration. This will enable this functionality, if a
defaults file is configured, else it will behave as it always has.
Merge pull request #224 from craigtracey/varsplugin

Add a default variable plugin