test/cli-integration/rbd: iSCSI REST API responses aren't pretty-printed anymore

test/cli-integration/rbd: iSCSI REST API responses aren't pretty-printed anymore

See https://github.com/ceph/ceph-iscsi/pull/263 and
https://github.com/pallets/flask/pull/2193.  Flask stopped
pretty-printing by default in 1.0:

  Change the default for JSONIFY_PRETTYPRINT_REGULAR to False.
  json.jsonify returns a compact format by default, and an indented
  format in debug mode.

Fixes: https://tracker.ceph.com/issues/57343
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 1cec9e83c02c366b5274739ae11297b6fca8584f)