rgw/CloudTransition: Delete cloud tiered objects by default

rgw/CloudTransition: Delete cloud tiered objects by default

Added a new option "retain_object" in tier_config which determines
whether a cloud tiered object is deleted or if its head object is
retained. By default the value is false i.e, the objects get
deleted.

XXX: verify that if Object is locked (ATTR_RETENTION), transition is
not processed. Also check if the transition takes place separately for
each version.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>