Document: OpenStack inventory script needs grizzly

The OpenStack Inventory script uses the OS-EXT-IPS:type parameter
to distinguish between fixed and floating IP addresses.
This was only added in the Grizzly release, see
https://bugs.launchpad.net/nova/+bug/1117784

Specify in the docs that this inventory script requires Grizzly
or greater.
Merge pull request #3050 from lorin/os-grizzly-only

Document: OpenStack inventory script needs grizzly