Make UpgradeLevelNovaCompute parameters consistent

Make UpgradeLevelNovaCompute parameters consistent

There is logic in nova-base.yaml that depends on the default for
this parameter being '', and the nova-compute service only needs it
set to auto during upgrade.  That will be done by [1] anyway, so it
doesn't matter what the default is.  It's also not clear to me that
the nova-compute task is even needed now that we're post-Ocata, but
that's not a change I feel comfortable making.

1: https://github.com/openstack/tripleo-heat-templates/blob/master/environments/major-upgrade-composable-steps.yaml

Change-Id: Iccfcb5b68e406db1b942375803cfedbb929b4307
Partial-Bug: 1700664