e2e-pool: Start real pool provisions after inventory added

Goal: reduce e2e-pool wallclock time by ~35m.

Problem Statement: When ClusterPool inventory
(ClusterDeploymentCustomization) testing was added to e2e-pool (4fddbe7
/ openshift#1672), it triggered ClusterPool's staleness algorithm such that we
were actually wasting a whole cluster while waiting for the real pool to
become ready. Grab a cup of coffee...

To make the flow of the test a little bit easier, we were creating the
real pool, then using its definition to generate the fake pool
definition -- which does not have inventory -- and then adding inventory
to the real pool.

But if you add or change a pool's inventory, we mark all its clusters
stale. So because of the flow above, when we initially created the real
pool without inventory, it started provisioning a cluster. Then when we
updated it (mere seconds later, if that), that cluster immediately
became stale.

Now, the way we decided to architect replacement of stale clusters, we
prioritize _having claimable clusters_ over _all clusters being
current_. Thus in this scenario we were actually ending up waiting until
the stale cluster was fully provisioned before deleting it and starting
over with the (inventory-affected) cluster.

Solution: Create the real pool with an initial `size=0`. Scale it up to
`size=1` _after_ adding the inventory.

hack/e2e-pool-test.sh

@@ -215,7 +215,12 @@ go run "${SRC_ROOT}/contrib/cmd/hiveutil/main.go" clusterpool create-pool \
 	--pull-secret-file="${PULL_SECRET_FILE}" \
   --image-set "${IMAGESET_NAME}" \
   --region us-east-1 \
-  --size "${POOL_SIZE}" \
+  # NOTE: We start with a zero-size pool and scale it up after we add the inventory.
+  # Otherwise, adding the inventory immediately makes the already-provisioning cluster
+  # stale, BUT it doesn't get purged until it's done provisioning. (Intentional
+  # architectural decision to prefer presenting a stale claimable cluster early vs.
+  # delaying until a non-stale one is available.)
+  --size 0 \
   ${REAL_POOL_NAME}
 
 ### INTERLUDE: FAKE POOL
@@ -234,7 +239,8 @@ oc get clusterpool ${REAL_POOL_NAME} -o json \
 NEW_CLUSTER_NAME=cdcci-${CLUSTER_NAME#*-}
 create_customization "cdc-test" "${CLUSTER_NAMESPACE}" "${NEW_CLUSTER_NAME}"
 oc patch cp -n $CLUSTER_NAMESPACE $REAL_POOL_NAME --type=merge -p '{"spec": {"inventory": [{"name": "cdc-test"}]}}'
-
+# Now we can scale up the pool so it starts creating clusters
+oc scale cp -n $CLUSTER_NAMESPACE $REAL_POOL_NAME --replicas=$POOL_SIZE
 
 wait_for_pool_to_be_ready $FAKE_POOL_NAME