Fix regression for old salt versions and bugfix for event_return_whitelist/event_return_blacklist options

The first two commits should fix a regression for old salt versions (introduced with #76) due to the fact that the included salt/formulas.jinja will fail to compile complaining that the variable "salt" doesn't exist. This is fixed first by not shadowing the global salt variable in all the templates and then by adding an explicit "with context" so that jinja copies over the context.

The last commit fixes a bug with the way the event_return_whitelist/event_return_blacklist options were managed.