qa/suites/upgrade/pacific-p2p: run librbd python API tests from pacific tip

qa/suites/upgrade/pacific-p2p: run librbd python API tests from pacific tip

This job installs librbd from v16.2.7, upgrades librbd to latest
pacific and then runs librbd python API tests from v16.2.7 against the
upgraded librbd.  This isn't expected to work in the general case
and is currently failing in TestImage.test_diff_iterate test because
it was adjusted to do the right thing in commit f8ced6d1fe66
("test/pybind/rbd: don't ignore from_snapshot in check_diff()").

Co-authored-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Yuri Weinstein <yweinste@redhat.com>