Use shell.cmd with shell.executable

Rather than passing a string to the shell task and then using
shell.args.executable to choose an explicit executable, use
shell.cmd to set the actual script content so that shell.executable
can be used to choose the shell.

Change-Id: Ia24a21ce54746a486ba39c4eb7c32bc8470e32d4

playbooks/release/tag.yaml

@@ -1,7 +1,8 @@
 - hosts: all
   tasks:
     - name: Run release scripts
-      shell: |
+      shell:
+        cmd: |
           set -x
 
           RELEASES_DIR=$HOME/src/{{ zuul.project.canonical_name }}
@@ -20,5 +21,4 @@
           RC2=$?
 
           exit $(($RC1 + $RC2))
-      args:
         executable: /bin/bash

roles/build-wheels/tasks/main.yaml

@@ -11,6 +11,6 @@
   become: yes
 
 - name: Build the wheel mirror
-  shell: ./scripts/wheel-build.sh {{ wheel_dir }} {{ wheel_python }}
-  args:
+  shell:
+    cmd: ./scripts/wheel-build.sh {{ wheel_dir }} {{ wheel_python }}
     executable: /bin/bash