Introduces escaping in Horizon/Orchestration

1) Escape help_text a second time to avoid bootstrap tooltip XSS issue

The "Description" parameter in a Heat template is used to populate
a help_text tooltip in the dynamically generated Heat form. Bootstrap
inserts this tooltip into the DOM using .html() which undoes any
escaping we do in Django (it should be using .text()).

This was fixed by forcing the help_text content to be escaped a second
time. The issue itself is mitigated in bootstrap.js release 2.0.3
(ours is currently 2.0.1).

2) Properly escape untrusted Heat template 'outputs'

The 'outputs' parameter in a Heat template was included in a Django
template with HTML autoescaping turned off. Malicious HTML content
could be included in a Heat template and would be rendered by Horizon
when details about a created stack were displayed.

This was fixed by not disabling autoescaping and explicitly escaping
untrusted values in any strings that are later marked "safe" to render
without further escaping.

Change-Id: Icd9f9d9ca77068b12227d77469773a325c840001
Closes-Bug: #1289033
Co-Authored-By: Kieran Spear <kispear@gmail.com>