Add storage interface to enabling-drivers doc
Add storage interface to enabling-drivers doc

Adds the storage interface to the enabling-drivers documentation
with example configuration, and notes that the interface is
explicitly an "opt-in" interface.

Change-Id: If471708c7827954a1935b4f6c0a312b9fa258c43
Partial-Bug: #1559691

openstack/ironic,pshchelo/ironic,openstack/ironic,pshchelo/ironic