ceph_rest_api.py: obtain and handle tell <osd> and pg <pgid> commands

Special treatment throughout for these commands:

* hack the help signature dump
* keep a 'flavor' per command to allow special handler() processing
* strip off 'tell/<target>' when constructing command
* allow multiple dicts with the same url for /pg?pgid=NN?cmd=XX
  (the parameters and get/put methods change)
* because of above, method must be validated in handler()
* validate the given OSD
* calculate target for command (mon, osd, pg)

Unrelated: make method_dict into global METHOD_DICT

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