vpp-checkstyle-verify-{stream}: add the step to API check

vpp-checkstyle-verify-{stream}: add the step to API check

This patch adds calling the crcchecker.py --check-patchset
in the initial checkstyle path.

(see change ID I5f13c0976d8a12a58131b3e270f2dc9c00dc7d8c)

It adds also:

- gerrit-refspec-parameter to be able to specify the patchset manually

- credentials-binding for a WebEx Teams bot - I had them added via IT-19800

- necessary plumbing to use these credentials. It is done by a minimal
  curl setup, which posts into room "VPP API check failures",
  which I created to be able to monitor the behavior and jump in
  to discuss and help to sort out the changes that may be problematic.

Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: I4833aa929044413a1ac08ab80668f25c0db8b3ff