The code that generates form data payloads for HTTP POST requests for

hosting services resulted in payloads that weren't easy to test against.

The fields were listed in dictionary iteration order, which varied

between Python 2 and 3. We also didn't have a great place to spy on

the boundary generator, since mimetools.choose_boundary() is no more

on Python 3.
This change improves testability by wrapping

mimetools.choose_boundary(), giving us a stable function to spy on,

and by sorting the data going into the payload so it's more predictable.

Unit tests pass.
Tested this along with other upcoming changes that test these payloads.