test and document use of fs.s3a.signing-algorithm

we suggest changing of fs.s3a.signing-algorithm as a way to deal with some auth problems, but there's no standalone section of what is available, or any tests trying to switch to different methods.

This is hampered by the fact that none of us have owned up to understanding all the choices available and the AWS docs not covering it...all we have is the source which sets up the options in >1 place

Proposed:

• add some tests of the actual valid options.
• document the values in the s3a docs
• and in core-default.xml