Provide a simple "hello world" example that includes details on using
the 'Resource' subclasses. References to 'clouds.yaml' are also moved
earlier in the doc since this is a pretty essential part of using
openstacksdk.

Change-Id: I6e7e6b5d9f54ec15b8e886b78649b50e02d38fc4
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>