Disable limestone provider

We will be working to upgrade the nodes from Xenial to Bionic over
the coming weeks, as well as performing hardware maintenance to
replace wore out SSDs.

Change-Id: I8f12e4467b8f7fbc61ecaf975c6ed6cb6f16c2f0

nodepool/nl02.openstack.org.yaml

@@ -81,7 +81,7 @@ providers:
         config-drive: true
     pools:
       - name: main
-        max-servers: 50
+        max-servers: 0
         labels:
           - name: centos-7
             min-ram: 8000