Decide which jobs should perform "extra work"

Right now we're "conserving resources" by lumping some unrelated work to
our jobs. A fine example is generating docs via Doxygen. This makes
sense because the CI system has a certain overhead, and it will only
become even more important with coverage testing that requires running
full build and a test suite. On the other hand, it is actively wasteful
to do this in all builds; normally it should be enough to build docs
just once, let's say when building with baseline GCC, and don't bother
with this extra work when doing a TSAN or ASAN+UBSAN build, or when
testing with another compiler.

This job therefore creates an env variable for our shell script,
JOB_PERFORM_EXTRA_WORK, which can be used by each project's build script
to decide whether or not to run bits such as `make docs`.

Change-Id: Id08afa2e75ec0c78435912eb7725b1b2bff296a2
diff --git a/zuul.yaml b/zuul.yaml
index 090d816..eaa0126 100644
--- a/zuul.yaml
+++ b/zuul.yaml
@@ -5,6 +5,7 @@
     run: playbooks/run-test-command/run.yaml
     vars:
       test_command: "ci/build.sh"
+      perform_extra_work: false
     nodeset:
       nodes:
         - name: w
@@ -26,6 +27,8 @@
     description: |
       Build on Fedora 29 with GCC.
     parent: run-ci-build-sh
+    vars:
+      perform_extra_work: true
 
 - job:
     name: f29-gcc-asan