Don't reinstall python-virtualenv on infra nodes

In the original change I said "for infra nodes, it shouldn't do
anything anyway ...".  Well that was pre-Fedora 26 :)

It seems that dnf > 2.0 now intentionally throws an error when trying
to explicitly install an ignored package.  Thus, as described in the
comment, take a simpler approach of skipping this on infra nodes.
pip-and-virtualenv in dib should have installed the latest pip,
virtualenv and setuptools, so we don't want to fiddle with that
anyway.

[1] https://review.openstack.org/#/c/338998/

Change-Id: Ib300b58377a0d0fe1bd7444c71acdb9a87dc033b
1 file changed