Allow "bin/drydock lease" to acquire many identical leases with "--count N"

Summary: Ref T13676. This makes it easier to create resource pressure without juggling a big pile of terminals.

Test Plan: Used bin/drydock lease --count 5 ... to acquire 5 leases.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13676

Differential Revision: https://secure.phabricator.com/D21800