ensure-twine: Check executable presence using shell+bin/bash

While making test with a node:
- with tox installed using "pip install --user" as a non-root user
- PATH modified using ~/.ssh/environment
- declared in nodepool to run as this non-root user

twine executable is not found:

using "command": twine is not found
using "shell" without /bin/bash: twine is not found
using "shell" with /bin/bash: twine is found!

Change-Id: Ia06218a830b5d44f04e0f10e0444be2cf9ad682e

roles/ensure-twine/tasks/main.yaml

@@ -1,5 +1,8 @@
 - name: Check for twine install
-  command: which twine
+  shell: |
+    command -v {{ twine_executable }} || exit 1
+  args:
+    executable: /bin/bash
   failed_when: false
   register: register_twine