Disable vexxhost for mirror work

Our vexxhost mirror in ymq is getting rogue RAs. Disable this region in
nodepool so that we can statically configure addresses on the mirror
without impacting jobs.

Change-Id: I42e332dcc36d0e2a7a29e38d916de679e3b36b9c

nodepool/nl03.openstack.org.yaml

@@ -166,7 +166,7 @@ providers:
     diskimages: *provider_diskimages
     pools:
       - name: main
-        max-servers: 50
+        max-servers: 0
         labels:
           - name: centos-7
             min-ram: 8000
@@ -244,7 +244,7 @@ providers:
             diskimage: centos-7
             key-name: infra-root-keys-2020-05-13
       - name: vexxhost-specific
-        max-servers: 20
+        max-servers: 0
         labels:
           - name: ubuntu-bionic-expanded-vexxhost
             flavor-name: 'osf-standard-16'
@@ -255,7 +255,7 @@ providers:
             diskimage: ubuntu-bionic
             key-name: infra-root-keys-2020-05-13
       - name: gpu
-        max-servers: 2
+        max-servers: 0
         labels:
           - name: centos-7-gpu
             min-ram: 8000