add bisect functionality to jenkins

build_bisect triggers builds and parses piglit results

jenkins.py has build_all method from build_jenkins.py, so bisect and
build_jenkins can both use it.  Extra arg provided to build(), so
specific piglit tests can be selected.  TODO: remove build_all from
build_jenkins.

dependency graph removes stale references in build_complete.  This
allows callers to mark a build complete when it's prerequisites have
not been built.

piglit-test/build.py takes an additional param to single out tests.