Add a flag to change when the May 15, 2018 hard fork activates

Summary:
This is key to be able to test the fork parameters.

Closes T245

Test Plan: Modify abc-p2p-fullblocktest to check for the activation.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Maniphest Tasks: T245

Differential Revision: https://reviews.bitcoinabc.org/D1159