This commit does a fairly major refactor of how the spec tests
are laid out. The main goal was to make it easier to run
a subset of the tests--e.g., the ability to only run tests
on a single OS via a simple rspec command.
The test logic is now defined in some shared examples in the
`support` directory. There are now spec folders for each
distro, which contain some stubs to include the shared examples
as well as a Vagrantfile for the particular distro.
Also, the system-default postgres package tests now run
successfully against the CentOS6 VM that is defined by the
Vagrantfile.
