Allow implementations to control whether drug orders require formulation

Per TRUNK-4437, we will allow drug orders to be created only with a concept (without referencing a specific formulation of a drug); however, some implementations will not want to allow this behavior.

Create a global property like "allow drug orders without specific formulation" that defaults to true. If set to false, then drug orders that do not reference a specific formulation (i.e., do not include a non-null drug_inventory_id) should be refused by the API.