Remove leftover option from sample config
Handle the case of no nova.ini file existing

The nova inventory modue will happily work with no config file,
except for how we're reading default values.
Add support for nova availability zones
Support multiple OpenStack clouds

It's entirely regular in OpenStack to have multiple cloud endpoints
that an infrastructure spans. Add support to nova.ini for expressing
that, and additionally add a few more groups and hostvars so that,
for instance, it would be sane to run commands just on one cloud.
Use global inventory for single host

Because we add some hostvars and whatnot in the server list, and
because we loop all the servers for the single case anyway, just
use the list codepath so that we don't have to duplicate additional
keys.
Record which cloud this is in the hostvars
Rearrange into class in prep for work on caching
Add caching for nova inventory