Use pip3 to install zuul on executors

The executors aren't using docker container images yet due to conflicts
with bubblewrap. This means we are still installing it directly on the
host using pip. Unfortunately we were using `pip` before which maybe
install to python2 and zuul doesn't run under python2. Address this by
explicitly using pip3.

Change-Id: I2ec551e8207e29ca420b09b8818154b9c32b47cf

playbooks/roles/zuul-executor/tasks/main.yaml

@@ -45,7 +45,7 @@
 
 - name: Install zuul
   shell:
-    cmd: pip install .
+    cmd: pip3 install .
     chdir: /opt/zuul
   when: zuul_repo is changed