ksgen: adds !env yaml tag to lookup env variables

!env enable looking environment variables. A default value can be
passed by using the list syntax !env ['VAR', default]. The main
difference between ansible lookup and this is that
- !env ['FOOBAR', none ] sets value to type 'None' where as
  '{{ ansible_env.FOOBAR | default(none) }}' is still a string
- gather_facts need not be set on local_hosts

Change-Id: I2da85f574480aca7113c815e03461c21c51ff7fb