Implement checks for installing Chef provisioner.

This commit ensures that the Chef provisioner is installed only if
provisioning is enabled, a Chef provisioner has been configured,
and Chef hasn't already been installed.

It also removes some unneccesary files.
Merge pull request #28 from smdahlen/chef-provisioner

Implement checks for installing Chef provisioner.
Refactor test runs to use Bundler environment.

This commit shifts running tests from the user's installed
Vagrant environment to using Bundler. This simplifies testing
with different Vagrant versions and ensures that no conflicts
occur with other installed plugins.