support passing extra arguments to bdist_wheel in build-python-release

Users of this role may want to pass extra arguments, such as the
--universal flag. Those arguments need to come after the bdist_wheel
argument to setup.py so that the command line processor understands them.

Change-Id: I38a3ef28b2c78821ec81a3f1adf0cfeef946d98f
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

roles/build-python-release/README.rst

@@ -7,3 +7,9 @@ Build sdist and wheel for Python projects.
 
    The python interpreter to use. Set it to "python3" to use python 3,
    for example.
+
+.. zuul:rolevar:: bdist_wheel_xargs
+   :default: ''
+
+   Extra arguments to pass to the bdist_wheel command when building
+   packages.

roles/build-python-release/defaults/main.yaml

@@ -1,2 +1,3 @@
 zuul_work_dir: "{{ zuul.project.src_dir }}"
 release_python: "python"
+bdist_wheel_xargs: ""

roles/build-python-release/tasks/main.yaml

@@ -1,4 +1,4 @@
 - name: Build a tarball and wheel
-  command: "{{release_python}} setup.py sdist bdist_wheel"
+  command: "{{release_python}} setup.py sdist bdist_wheel {{bdist_wheel_xargs}}"
   args:
     chdir: "{{ zuul_work_dir }}"