Extend support for Nova / Neutron resources in "rally show" command

This blueprint aims to add support for getting neutron / nova resources using "rally show" command.
Currently "rally show" supports only network, security group, flavor and image amongst nova / neutron resources.
https://github.com/openstack/rally/blob/master/rally/cli/commands/show.py

The blueprint extends this to subnet, router, floatingip and VMs ( which are common resources that a user outside OpenStack should be able to view )
Once support for above resources is available, we can extend it further based on needs.