Remove heat::db class

heat::db is different to every other project's puppet modules.  All of
nova, glance, keystone and cinder take a "sql_connection" parameter to
the "main" (project name) class.

This removes heat::db and adds it into init.pp to maintain parity with
those projects.  This seems more logical than adding it to engine.pp;
since heat-engine is only using the DEFAULT section of heat.conf.

Change-Id: I4584cdb12d1f9e624228e6ee34b4bcfbf649a12e