Provide full path for iptables-clear script

Make use of the `playbook_dir` variable to provide the full path to
iptables-clear.sh within the test-prepare-host playbook.

Change-Id: I9f4cd8856d0e568ce0377e75c6c02df188480e01

test-prepare-host.yml

@@ -17,7 +17,7 @@
   hosts: localhost
   pre_tasks:
     - name: Clear iptables rules
-      shell: ./iptables-clear.sh
+      shell: "{{ playbook_dir }}/iptables-clear.sh"
     - name: Update apt cache
       apt:
         update_cache: yes