Limit zuul stop/start playbooks to zuul02

Rather than restarting the whole scheduler group, just restart
zuul02, which is our only production scheduler.  That will allow us
to boot zuul01 as a secondary scheduler and manually add/remove it
for testing.

Once we can reliably run two schedulers, we can revert this change.

Change-Id: I5518ea1d3a6a1d48460b0436d4d1eaf9d52b7ddb

playbooks/zuul_start.yaml

@@ -1,6 +1,6 @@
 # Run this with "-f 20"
 
-- hosts: 'zuul-scheduler:!disabled'
+- hosts: 'zuul02.opendev.org:!disabled'
   tasks:
     - include_role:
         name: zuul-scheduler

playbooks/zuul_stop.yaml

@@ -1,6 +1,6 @@
 # Run this with "-f 20"
 
-- hosts: 'zuul-scheduler:!disabled'
+- hosts: 'zuul02.opendev.org:!disabled'
   any_errors_fatal: true
   tasks:
     - include_role: