Add new “salt.formulas” state to manage salt formulas

This state downloads formulas from git repositories and ensures
that they are recorded in the file_roots setting.
Merge pull request #76 from rhertzog/formulas

Add new “salt.formulas” state to manage salt formulas
Merge pull request #77 from rhertzog/misc-bugfixes

FIxes for nodegroups/log_granular_levels settings and small cleanups
Avoid “set salt” jinja calls that mask the usual salt variable

Most include do not expect salt to be something else than the usual salt
variable giving access to all the salt modules. Instead we use cfg_salt.
And for consistency we rename the master/minion variables to
cfg_master/cfg_minion too.
Import the salt/formulas.jinja template with context

This is only needed for salt versions older than 2014.7 that do not
define the global "salt" variable in such context.
Fix the handling of event_return_whitelist/event_return_blacklist

If those options are set in pillar data, the jinja template
salt/files/master.d/_defaults.conf would fail to compile trying to
evaluate non-existing variables.

Replace those variables with the corresponding dictionnary entries.