Reporting: properly close db connection

Close the db connection at the end of each DjangoORM import, not when
the reporting collector shuts down.  The collector may not have even
opened a connection, in the case of a storage backend other than
DjangoORM.

Fixes #157
Core: close all database connections at the end of XML-RPC requests
cleared pylint error on PluginDatabaseModel metadata options
bcfg2-report-collector: better error messages when failing to daemonize
Core: better error messages when altsrc bind fails