rgw/CloudTransition: Do not allow data pool for tier type storage classes

rgw/CloudTransition: Do not allow data pool for tier type storage classes

Tier type storage classes should not be allowed to have data
pools

& few other fixes/cleanup stated below -

* If the tier_targets are not configured, do not dump them in
the 'zonegroup get' command.

* If not configured, by default a bucket of below name convention -
"rgwx-$zonegroup-$storage_class-cloud-bucket"

is created in the remote cloud endpoint to transition objects to.

* Rename config option 'tier_storage_class' to 'target_storage_class'.

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