jinja2 templates: Enable the 'do' and 'loopcontrols' extensions

As discussed in https://github.com/saltstack/salt/pull/3186#issuecomment-17097427

This commit enables 2 extensions in jinja2 that then provide 3 very useful constructs:
  * do
  * break
  * continue

While the intention of jinja2 is generally to encourage more code and logic in the app and less in the template, I believe in salt's case where the user is just writing templates, opening these up is a good idea.