R1:e946e7cebc2b

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…