Review the logic for add/delete flows

[ajoaoff]

Original issue opened by @viniarck at kytos#58.

Currently, if the dpid is None, it's installing flows in all switches: https://github.com/kytos/flow_manager/blob/master/main.py#L52-L59

The same logic is also being applied for the delete method. So, this issue is to review this logic and assess a better one. Maybe, a new method which could iterate over all switches, but stop relying on this 'None' to consider the whole list.