Add a "--gently" flag to phd stop and phd restart

Summary:
In the cluster, the box has a ton of stuff that "looks like a daemon" beacuse it is some other instance's daemon.

Stop phd restart from complaining about this if given a "--gently" flag, which is like the opposite of "--force".

(I'll make it stop --force at the beginning of a whole-box restart to kill stragglers.)

Test Plan: Ran bin/phd restart --gently, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11784