Since gnocchi::db class was introcued[1], we have 2 parameters,
gnocchi::database_connection and gnocchi::db::database_connection,
to configure the same database_connection parameter.
Let's deprecate the one in gnocchi class, so that we can have
only one parameter to set the one parameter.
[1] 4d64bd45a7
Change-Id: I6fb80bc63a9f7b370f9349681327a50d705d7514