s3tests: add tests for CopyObject tagging-directive

Add tests to validate that CopyObject honors x-amz-tagging-directive.

New tests:
- test_object_copy_retaining_tagging: default copy keeps the source tags
- test_object_copy_tagging_directive_copy: COPY keeps the source tags
- test_object_copy_tagging_ignored_without_directive: a tagging header is ignored without a REPLACE directive
- test_object_copy_replacing_metadata_and_copying_tagging: metadata REPLACE keeps the source tags
- test_object_copy_replacing_tagging: REPLACE applies the new tag set
- test_object_copy_replacing_tagging_with_none: REPLACE with no tags clears them
- test_object_copy_to_itself_replacing_tagging: retag onto the same key
- test_copy_object_replacing_tagging: replaced tags replicate across zones (multisite)

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>