Use platform_family instead of platform

As agreed on the mailing list:
https://groups.google.com/forum/#!topic/opscode-chef-openstack/HEL3KqBQIUM

This will allow the ibm_powerkvm platform to be recognized.

Change-Id: Iebf2aad9f3b4649fb5c27d2fc93ee60c67e42737
Partially-Implements: blueprint add-ibm-powerkvm-enablement
Implements: blueprint platform-family