Add ansible raw task note, add apt-get update

Add note about raw install
Add apt-get update before install python

TrivialFix
Closes-Bug: #1618297

Change-Id: I08372dcfe71dff8c0a50a421788309b642e2a521

ansible/roles/baremetal/tasks/pre-install.yml

 ---
-
+# NOTE: raw install is required to support cloud images which do not have python installed
 - name: "install python2 and python-simplejson"
   become: true
-  raw: "yum install -y python python-simplejson || apt-get install -y python2.7 python-simplejson"
+  raw: "yum install -y python python-simplejson || (apt-get update && apt-get install -y python2.7 python-simplejson)"
 
 - name: gather facts
   setup: