Switch scheduler drivers to load based schedulers

Switch scheduler drivers to load based schedulers

Cloud deployed at scale most likely will use these scheduler
drivers because they allow a fairer resource allocation compared
to chance schedulers (which randomly place resources on the hosts).

Because of their importance, it's only wise to test them in
the gate on a continuous basis, so that we do not get surprised
by accidental regressions.

Rather than pushing this down through devstack-gate/project-config
patches, this chance alters the default of the scheduler
drivers, so that users can also pick these up out of the box.

This means that after an upgrade they would observe a change in
the scheduling behavior, if they relied on the default config.

DocImpact
UpgradeImpact

Closes-bug: #1494667

Change-Id: I5927914cb88eff66bc7a045340ff68cb8da95ad6