In my environment, my computes have "untagged" venvs, so the path
to the venv on nova_compute hosts has "nova-untagged" instead of
"nova-<tag>", meaning this tasks breaks because it is delegated from
computes to a conductor host, but does not use the conductor vars to
reference the location of nova's venv.
Change-Id: I3967cbad0cbcc2214db1ca1f2225f3d140fd1774