add some docs for how to do complex custom commands

website/source/docs/provisioners/ansible.html.md.erb

@@ -70,7 +70,9 @@ Optional Parameters:
     ```
 
 -   `command` (string) - The command to invoke ansible. Defaults to
-    `ansible-playbook`.
+    `ansible-playbook`. If you would like to provide a more complex command,
+    for example, something that sets up a virtual environment before calling
+    ansible, take a look at the ansible wrapper guide below for inspiration.
 
 -   `empty_groups` (array of strings) - The groups which should be present in
     inventory file but remain empty.
@@ -354,3 +356,32 @@ $ ssh-add -D
 
 We recommend against running Packer as root; if you do then you won't be able
 to successfully run your ansible playbook as root; `become: yes` will fail.
+
+### Using a wrapping script for your ansible call
+
+Sometimes, you may have extra setup that needs to be called as part of your
+ansible run. The easiest way to do this is by writing a small bash script and
+using that bash script in your "command" in place of the default
+"ansible-playbook".  For example, you may need to launch a Python virtualenv
+before calling ansible. To do this, you'd want to create a bash script like
+
+``` sh
+#!/bin/bash
+source /tmp/venv/bin/activate && ANSIBLE_FORCE_COLOR=1 PYTHONUNBUFFERED=1 /tmp/venv/bin/ansible-playbook "$@"
+```
+
+The ansible provisioner template remains very simple. For example:
+
+``` json
+  {
+    "type":  "ansible",
+    "command": "/Path/To/call_ansible.sh",
+    "playbook_file": "./playbook.yml"
+  }
+```
+
+Note that we're calling ansible-playbook at the end of this command and passing
+all command line arguments through into this call; this is necessary for
+making sure that --extra-vars and other important ansible arguments get set.
+Note the quoting around the bash array, too; if you don't use quotes, any
+arguments with spaces will not be read properly.