Actually generate pip freeze outputs

the shell script used is actually being run thru 'sh', not bash,
which does not understand "[[" test operators.

Explicitly run this script with /bin/bash instead.

Change-Id: I551d2631bcb6aef49550d69b3830ffcb509abfb7

roles/capture-system-logs/tasks/main.yaml

@@ -1,6 +1,7 @@
 # TODO(andreaf) Make this into proper Ansible
 - name: Stage various logs and reports
   shell:
+    executable: /bin/bash
     cmd: |
       sudo iptables-save > {{ stage_dir }}/iptables.txt
       df -h > {{ stage_dir }}/df.txt