Docs: add a note about rackspace API keys
Docs: add a note about rackspace API keys

Some users are forced to use these instead of passwords (whether because
they use 2-factor auth or by policy). Document it so they know how.

Change-Id: I558c2e8d3e8b0fad0a96a361232f14443e82a35f

dtroyer/os-client-config,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack/os-client-config,dtroyer/python-openstacksdk,openstack/python-openstacksdk