re-structure cfg-buildslaves manual page

* put documentation for supported latent buildslaves into separate
  documents
* polish a bit each page to give an overview of what they provide

Fixes ticket:2861
Merge sa2ajj/buildbot:bugfix-2861 (PR #1232)
test for and consume warnings about legacy GSP
correctly set the path module for p4 tests on windows

This fixes #2916.
Merge pull request #1237 from djmitche/bug2916

correctly set the path module for p4 tests on windows
Merge pull request #1235 from djmitche/gsp-warnings

test for and consume warnings about legacy GSP