Create each Site's network with its own subnet

The deploy failed on hngmohali with 'all predefined address pools have been
fully subnetted': every 172.17-172.30/16 is in use, 172.31/16 is the host's own
VPC so Docker will not allocate from it, and all sixteen 192.168.0-240/20 blocks
are taken. Docker therefore refuses a network created without a subnet.

Each Site now names a free network_subnet (10.210.0.0/16, one /24 each), the role
looks the network up first and only creates it when it is absent and with that
subnet, and it fails with an explanation when a Site declares none. Existing
networks are left alone, because Docker cannot change a subnet in place and the
Sites already running have theirs. The test checks every Site declares one, and
the deployment readme says a new Site needs a free subnet.

deploy.sh now deploys every Site by default, with SITE=<name> to narrow it, which
is what the local edit was reaching for.

deploy.sh

@@ -40,7 +40,11 @@ if [[ -n "${SKIP_DEPLOY:-}" ]]; then
   exit 0
 fi
 
-SITE="${SITE:-hopsngrains}"
-echo "==> deploying $SITE"
 cd ansible
-ansible-playbook --inventory=hosts playbook.yml --limit "$SITE" -e "tag=$TAG"
+if [[ -n "${SITE:-}" ]]; then
+  echo "==> deploying $SITE"
+  ansible-playbook --inventory=hosts playbook.yml --limit "$SITE" -e "tag=$TAG"
+else
+  echo "==> deploying every Site"
+  ansible-playbook --inventory=hosts playbook.yml -e "tag=$TAG"
+fi