Toggleable dependencies + HTTPS URL + minor improvements.

Toggleable dependencies:
Adding parameters that can be passed to dependent roles (nginx, redis, python) and enable/disable them. A possible use case is when nginx config is handled elsewhere.

HTTPS URL:
SITE_URL_PREFIX in config.py.j2 had "http://" in it, which made some requests fail when sentry is running on a "https://" URL. Added a boolean parameter for that.

Minor improvements:
Adding MySQL requirements.