qa/suites/upgrade/pacific-x/rgw-multisite: specify distro

qa/suites/upgrade/pacific-x/rgw-multisite: specify distro

instead of using the default / existing distro, specify the distro for
testing. this change prevents us from using bionic for testing the
builds. this has two consequences:

* so we are one step closer to a non-bionic world.
* avoid building packages with PPA repo which *might* introduce runtime
  dependencies on 3rd party runtimes provided by PPA repo.

see also: https://tracker.ceph.com/issues/50218

Signed-off-by: Kefu Chai <kchai@redhat.com>