Add a "--force" argument to "bin/config done"

Summary:
Ref T11922. When we deploy on Saturday I need to rebuild all the cluster indexes, but some instances won't have anything indexed so they won't actually trigger the activity.

Add a --force flag that just clears an activity even if the activity is not required.

Test Plan: Ran bin/config done reindex --force several times.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11922

Differential Revision: https://secure.phabricator.com/D16970