Convert system tests to use rspec-system

This patch ports all of the existing system tests to use rspec-system instead.

To assist with this conversion some patches were made to fix OS compatibility
where necessary. We also added an ensure parameter to the postgresql::server
class to assist with removing PostgreSQL configuration to aid with testing
cleanups.

The documentation has been updated to indicate test usage with rspec-system,
we've also renamed the 'tests' directory to 'examples'.

Signed-off-by: Ken Barber <ken@bob.sh>