Add an only_if option to vars_prompt to make prompts conditional

Sometimes you may want to allow variables through host_vars or inventory, but prompt for a value if it is not set or if the value does not conform to something specific. This option allows you to specify when you want to offer a prompt.

This patch also moves check_conditional to utils, and adds an is_unset() function which is nicer to read:

    only_if: "not is_set('${var}')"

vs

    only_if: "is_unset('${var}')"

lib/ansible/utils.py

@@ -93,6 +93,13 @@ def is_failed(result):
 
     return ((result.get('rc', 0) != 0) or (result.get('failed', False) in [ True, 'True', 'true']))
 
+def check_conditional(conditional):
+    def is_set(var):
+        return not var.startswith("$")
+    def is_unset(var):
+        return var.startswith("$")
+    return eval(conditional)
+
 def prepare_writeable_dir(tree):
     ''' make sure a directory exists and is writeable '''