Tweak ansible default vars plugin to work with our usage.

In ansible, the `hash_behaviour = merge` setting does
not apply to vars which come from plugins.

This change adds a monkey-patch to cause this setting to be
honored.

The monkey patch should be removed if this change can
me merged into upstream ansible.
Merge pull request #231 from blueboxgroup/defaults

Tweak ansible default vars plugin to work with our usage.