Allow indentation in stack configuration files

[cmhamill]

Accommodate leading whitespace in stack configuration files. This allows the Jinja to be formatted in a traditional, nested indentation style.

For example:

{% for role in pillar.get('roles', []) %}
    role/{{ role }}.yml
    {% if stack.meta[role].foo %}
        extra/thing.yml
    {% endfor %}
{% endfor %}

[cmhamill]

@bbinet I've update it to strip whitespace from item once.

[bbinet]

Thank you @cmhamill