Provide a custom provisioner for CentOS / RHEL based ISO images

[hferentschik]

Using libmachine a so called provisioner is called after the initial creation of the VM. This provisioner takes care of installing and configuring Docker.

At the moment the default Red Hat provisioner is run for CentOS and RHEL images. We really want to customize this. To do so one needs to implement a custom Detector which in turn returns the appropriate Provisioner.

The idea is basically to just handle CentOS / RHEL differently and in other case delegate/mirror the behavior of StandardDetector.

[hferentschik]

Resolved via pull request #181