Add nodeset ubuntu-bionic-2-node for bionic multinode testing
Add nodeset ubuntu-bionic-2-node for bionic multinode testing

Given that Bionic is the Ubuntu LTS release standard for the Stein
cycle, there should be a 2-node nodeset for it.  This will enable
multinode grenade testing on python3 to be done correctly.

Change-Id: I4a756f1f284a6967853da2dc34e477b2ba7b2340
Needed-By: https://review.openstack.org/622612

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs