Update cinder content for Juno

I updated the cinder content in the installation guide for Juno
as follows:

1) Clarified verify operation section.
2) Added instructions for attaching a volume to an instance. We
   create it, so might as well complete the process and attach
   it to an instance.

Change-Id: I658193599bb7ab91407e0180f686e2e3c6b2bf61