qa/tasks/cephadm.py: only pass --objectstore when not bluestore

qa/tasks/cephadm.py: only pass --objectstore when not bluestore

This commit ensure that we only pass --objectstore argument to
cephadm's add/apply OSD command only when the value is not the
default value, bluestore.

This is done to ensure older ceph releases, like Squid and Tentacle
do not fail, as --objectstore argument was only added in Umbrella.

Fixes: https://tracker.ceph.com/issues/75731
Signed-off-by: Shraddha Agrawal <shraddha.agrawal000@gmail.com>