Extend the devstack job so that it can support both single and multinode
cases. Multinode mode require extra settings in devstack configuration,
some of which as subnode specific, some controller specific.
Also keep a simple devstack-multinode job defined for now so we can run
a multinode job in devstack gate, until the full tempest multinode job
is ready to match the old
gate-tempest-dsvm-neutron-multinode-full-ubuntu-xenial-nv.
Fixing multinode also requires sharing the CA configuration between
controller and peers, overlay network configuration for communication
between virtual machines and running discover_hosts for nova after the
subnode has been setup.
The extra orchestration required for multinode is encoded in a
dedicated role to allow for jobs in other repos to re-use it.
This backport is slightly modified from the original, since master does
not use the test-matrix anymore, while on stable branches we still rely
on it.
Change-Id: I2dcbd9bdb401860820e655d97aa3c4775af2827f
6f377c88f7