Improvements from comments in https://github.com/3scale/porta/pull/555
I will address all comments, either if we decide to do everything or not

Changes requested in the other PR

• [Refactor] Rename Scope has_paid_on -> have_paid_on. Because has kinda implies that it is one instance, not a collection like it is the case

• MAX_UNPAID_TIME should be configurable. Requested in https://github.com/3scale/porta/pull/555#discussion_r251497355 & https://github.com/3scale/porta/pull/555#discussion_r251485309
  It is also done for the related values.

• The return if ThreeScale.config.onpremises in both workers should be done by rolling_updates. Requested in https://github.com/3scale/porta/pull/555#discussion_r251493628
  It depends on https://github.com/3scale/deploy/pull/537

• Avoid the term enterprise in the code if possible and make it configurable. Requested in https://github.com/3scale/porta/pull/555#discussion_r251484561

• Rename without_suspended to either unsuspended or not_suspended.
  Requested in https://github.com/3scale/porta/pull/555#discussion_r252300336 & https://github.com/3scale/porta/pull/555#discussion_r252300189
  I am not sure if this is a good idea of change because imo it should be consistent with the scope of not_deleted state
  https://github.com/3scale/porta/blob/8474333f885e82638aecbb2b047302e7f19444ea/app/models/account/states.rb#L97
  So for me either we change both or none