Remove stop nova processes from upgrade-nova

stop-base already stop services, so don't need to do it again in
upgrade-nova. This is needed So we can keep the old nova-compute up for
rolling upgrade testing. Instead of killing the entire screen process
kill all the services inside of it.

stop-base is the preferred place to manage the stop logic, because it
has access to the base devstack's stop-* commands.

Change-Id: Ifa2b6305f1f6bbe41407ca12a0f3cbc99b09f301
Add rolling upgrade test for nova-compute

Add rolling upgrade test for nova-compute. Nova wants to support the ability
to run stable/havana nova-compute with icehouse everything else. This sets
up a test for that.

When $DO_NOT_UPGRADE_SERVICES contains "n-cpu", the final environment
will be trunk everything except havana nova-compute.  Although
$DO_NOT_UPGRADE_SERVICES takes a list of services other parts of grenade
have to be fixed to support values besides "n-cpu", but a list is used
because in the future we want to support adding other values to this
list.

Change-Id: Ifb733ee2fe0c5bba61b43ae19dca7040c12f7893