[user-guide]Use "openstack" to replace "nova" command
[user-guide]Use "openstack" to replace "nova" command

This patch:
- Use "openstack" to replace "nova" command when gathering parameters to launch an instance
- update the "note" of available security groups

Implements: blueprint use-openstack-command
Change-Id: Ia8bc262e20efd19f2ab819489368a5d7a42fc243

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals