Bug 1405412 - Pre: Allow toolchain task images to not cache tc-vcs. r=dustin

Not all Docker images are configured for tc-vcs caches: in particular,
android-build is not configured.  Until we fully remove tc-vcs, this
will let toolchain tasks use non-tc-vcs caching images.

MozReview-Commit-ID: CYSdn2kpF3S

--HG--
extra : rebase_source : 87dec4fdb32290ef396cda7f92da6bd688bc5b7b

taskcluster/taskgraph/transforms/job/toolchain.py

@@ -47,6 +47,9 @@ toolchain_run_schema = Schema({
         'internal',
     ),
 
+    # If true, tc-vcs will be enabled.  Not supported on Windows.
+    Required('tc-vcs', default=True): bool,
+
     # Paths/patterns pointing to files that influence the outcome of a
     # toolchain build.
     Optional('resources'): [basestring],
@@ -120,6 +123,7 @@ def docker_worker_toolchain(config, job, taskdesc):
     if not any(artifact.get('name') == 'public/build' for artifact in artifacts):
         docker_worker_add_public_artifacts(config, job, taskdesc)
 
+    if run['tc-vcs']:
         docker_worker_add_tc_vcs_cache(config, job, taskdesc)
     docker_worker_add_gecko_vcs_env_vars(config, job, taskdesc)
     support_vcs_checkout(config, job, taskdesc, sparse=True)