bucket policy: improve the helper functions used for tagging upwards
bucket policy: improve the helper functions used for tagging upwards

- Improve `make_json_policy` to support conditionals in policy
- Move the helper functions for creating policies up so that bucket
  policy tests can use these
- add bucket-policy attribute to the tagging tests using policy

Signed-off-by: Abhishek Lekshmanan <0731fdeb307910eeaae4e58897b7baaba45e054b@suse.com>

timuralp/s3-tests,jmunhoz/s3-tests,robbat2/s3-tests,andrewgaul/s3-tests,ceph/s3-tests,robbat2/s3-tests,ceph/s3-tests,jmunhoz/s3-tests,andrewgaul/s3-tests,timuralp/s3-tests