qa/suites/rbd: override extra_system_packages directly on install task

qa/suites/rbd: override extra_system_packages directly on install task

[1] and [2] added support for applying extra_system_packages overrides
directly on install task, but at the same time broke our long standing
workaround where we sneaked extra_system_packages directive in through
an override on ceph task.  This is likely getting addressed in [3], but
it's better to not rely on this odd feature in the first place.

[1] https://github.com/ceph/teuthology/pull/1941
[2] https://github.com/ceph/teuthology/pull/1943
[3] https://github.com/ceph/teuthology/pull/1942

Fixes: https://tracker.ceph.com/issues/66232
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>