Allow setting private flag in nova.ini

It's useful for people at times to express that they always need to
connect to the private, not the public, interface.
Move nova connection code to shared library
Use OpenStackCloud in nova modules
Prep OpenStackCloud for more than Nova
Replace factory method with function
Use nova_client in nova_compute
Split openstack options out into the module_utils
Rename OpenStackAnsibleException
Add glance support to OpenStackCloud
Use glance in nova_compute

If the glance endpoint is available, it's more efficient to use it
than to use the nova endpoint passthrough. Use the image listing
routines in OpenStackCloud to try glance first, then nova.
Rename service_type to nova_service_type

service_type is only settable at the moment in the inventory script,
and even there it's only for backwards compatability reasons. While we
want to be consistent, we don't want to _add_ backwards compat issues
into new places ... so make sure it's an optional class constructor
parameter and don't add it to any argument specs.
Make keystone_user use OpenStackCloud
Enable nova to process keystone tokens
Add support for OpenStack volumes
Fix an editor derp in the magic comment

Clearly, someone derped at one point with their editor on top of
the warning about the magic imports, and then that derpage has been
copied willy-nilly across all of the OpenStack things. This doesn't
have a functional impact, but since it's telling people where to
look for better understanding of magic, it should point to the
right place.
Add volume information into nova inventory
Move config file processing into openstack library