A whole set of Glance functionality is not usable under uwsgi, including any
of the more powerful async import, customization, and copying functions.
In order to facilitate writing and running tempest tests for these features
in all environments covered by the various jobs across all the projects that
include Glance, we should default to this deployment method.

It is still possible to deploy glance in uwsgi mode by setting the flag to
False, and we can do that for some jobs to make sure that it continues to
work. However, the default should be what we expect deployers will use,
which is standalone mode.

Depends-On: https://review.opendev.org/741479
Change-Id: I141acab2a07a4eebd8d850f900058bc8cbf9c7bf