Add a --force command to phd start

Summary:
Ref T7352. This isn't wildly useful for us but seems generally reasonable, can be helpful with testing, and @hach-que has a use case for it.

The only reason we issue this warning is to prevent user error; you can still launch all the daemons with phd launch manually and daemons all use locks to protect critical regions.

Test Plan: Ran phd start --force a bunch, saw zillions of daemons.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, hach-que

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11861