Add description of custom resource classes
Add description of custom resource classes

Add the description about custom resource classes and
overriding standard resource classes in the "Flavors" document.

Change-Id: I5b804db70d229696e7b7c5b5db16946cf1f1c49f
Closes-Bug: #1800663

klmitch/nova,klmitch/nova,openstack/nova,rahulunair/nova,mikalstill/nova,openstack/nova,rahulunair/nova,mahak/nova,mahak/nova,mikalstill/nova,mahak/nova,mikalstill/nova,klmitch/nova,rahulunair/nova,klmitch/nova,openstack/nova