First OSP instructions clean-up destroys also the initial bastion

[ericzolf]

Describe the bug
The file docs/First_OSP_Env_walkthrough.adoc contains at the end instructions which don't destroy only the nodes deployed by the developer but also the one initial bastion from the sandbox deployment.

To Reproduce
Follow the steps from the documentation...

Expected behavior
The initial bastion should remain untouched by the clean-up command so that the developer can destroy their environment and re-deploy it again and again, and continue to use the initial bastion to come into the environment and check potentially what might have gone wrong using openstack commands.