ceph_argparse.py: validate(): allow args to be lists

Multivalued arguments are passed in as lists.  Allow this, although
handle it somewhat cheesily by just extracting the first element
when encountered and pushing the rest back onto the front of the
arguments as non-list args.  This lets the looping/counting/validation
continue as normal.

Also, some comment cleanup and a nested-if removal because validate()
is looking pretty ugly.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph-rest-api: accept multivalued params now that validate() can take em

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph-rest-api: response format handling cleanup

Don't use keyword format for variables, allow plain format,
return response with enclosing JSON or XML so that the output
and status messages can be sent back independently (the equivalent
of stdout and stderr in the CLI).

Signed-off-by: Dan Mick <dan.mick@inktank.com>
rest test: wip