Make canceling a queue item a POST request.

Needed by Jenkins 1.502.

(Also don't print the results of error messages so they don't
clutter stdout, but leave them there as a debug option
comments because they are very handy when debugging
Jenkins API issues.)

Change-Id: I2e1dcc1ac2344b1eb9f66292b6fd30900cb45e6d
Reviewed-on: https://review.openstack.org/22579
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins