ci-operator/step-registry/ipi/conf/azure: Region from Boskos lease

To help avoid errors like "we randomly assigned more Azure centralus
clusters than we had capacity for and they died on quota limits".
From [1]:

> A test may access the name of the resource that was acquired using
> the ${LEASED_RESOURCE} environment variable.

[1]: https://steps.ci.openshift.org/help/leases#static

ci-operator/step-registry/ipi/conf/azure/ipi-conf-azure-commands.sh

@@ -6,18 +6,8 @@ set -o pipefail
 
 CONFIG="${SHARED_DIR}/install-config.yaml"
 
-case $((RANDOM % 8)) in
-0) AZURE_REGION=centralus;;
-1) AZURE_REGION=centralus;;
-2) AZURE_REGION=centralus;;
-3) AZURE_REGION=centralus;;
-4) AZURE_REGION=centralus;;
-5) AZURE_REGION=centralus;;
-6) AZURE_REGION=eastus2;;
-7) AZURE_REGION=westus;;
-*) echo >&2 "invalid Azure region index"; exit 1;;
-esac
-echo "Azure region: ${AZURE_REGION}"
+REGION="${LEASED_RESOURCE}"
+echo "Azure region: ${REGION}"
 
 cat >> "${CONFIG}" << EOF
 baseDomain: ci.azure.devcluster.openshift.com
@@ -29,5 +19,5 @@ compute:
 platform:
   azure:
     baseDomainResourceGroupName: os4-common
-    region: ${AZURE_REGION}
+    region: ${REGION}
 EOF