Gemfile: Upgrade to ChefSpec 3.
spec_helper: Use chefspec/berkshelf.

Replaces the custom cookbook vendoring with the Berkshelf support
added in ChefSpec 3.
spec tests: Upgrade custom matchers.

Rework the python_virtualenv matcher defined in default_spec by adding
it to spec/support/matchers.rb using the pluggable matcher framework.
spec/default: Upgrade to ChefSpec 3.
Update support files.

Various small and sundry updates to the Guardfile and Vagrantfile.
Also adds a rspec options file.
Merge branch 'feature/upgrade-chefspec' into develop