Fix note on specifying a specific host for nova
Fix note on specifying a specific host for nova

The parsing goes like this in Nova:

1. --availability-zone ZONE (no host or node)
2. --availabllity-zone ZONE:HOST (no node)
3. --availability-zone ZONE::NODE (no host)
4. --availability-zone ZONE:HOST:NODE

So we need to fix the docs to match case #2.

Change-Id: Iedb8d221d0a33f18a7e4e10dff7b35823eef90a7
Closes-Bug: #1673252

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals