buffet: Support multiple proxies for a command.

This is necessary to support Cloud API updates on command
changes.

BUG=None
TEST=cros_workon_make buffet --test&&manual

Change-Id: I09149d02c0564aa89c74bd66356b63547c17724f
Reviewed-on: https://chromium-review.googlesource.com/226533
Tested-by: Anton Muhin <antonm@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Anton Muhin <antonm@chromium.org>