Fix up nova inventory to be modern

Update the nova inventory module to be more like the ec2 and rax modules. Notably:

* added support for nova.ini to express multiple clouds that the inventory should span
* added caching similar to the ec2 module
* aligned authentication and floating ip handling with the other nova modules
* added "--private" flag so that one can specify "please use the private IP (fixed IP) of the instance as ansible_ssh_host
Thank you very much for your review of this! Will fix.