Since some tests from the plugin require list of nodes be available
a discover_nodes() function was implemented, by using openstack API.
This will ensure that discovering nodes will work properly on
environments deployed with any installer.
Also, adjusted multicast tests in order to prevent self-rebooting
of the tempest node on a single-node environment.
Change-Id: I80dd0ba855a63952d12214a4e7e9fd9c334e312a
8ac79db86c