Properly identify Raspbian
Use debian_service as service provider for Raspbian
Fix traceback in service.py when OS not identified

If the OS was not properly identified, salt/modules/service.py is used
as a fallback provider for the service module/state. However, the way it
looks up the location of the initscripts directory results in a
traceback if the OS grain isn't in the lookup dict.

This commit simplifies the lookup dict and has the module use
/etc/init.d as the location for initscripts if the 'os' grain is not in
the lookup dict, fixing this traceback.
Merge pull request #7434 from terminalmage/issue7371

Properly identify Raspbian to fix service/pkg support