Update cinder generate_sample script

Update cinder generate_sample script

Cinder's generate sample script has fallen a
bit out of date and was no longer working properly
in local venv.

This patch updates it with a working version that
reads the rc file and makes some other updates.

Even better, we'll update the tox.ini file here so
that it actually works, and finally add some more
info to the error message when update check fails
to make is obvious how to run these appropriately.

Change-Id: I2389910d8ece6f737a9609ce4aa05263d9293e14