Fix telemetry integration gate jobs

Since https://review.openstack.org/#/c/281779/2 have been
merged the telemetry integration job is broken.

Unfortunatly, it can't be fixed on our side, because we have to
rename SERVICE_TENANT_NAME in many devstack plugin, we can't merge thing
until all plugins have been fixed.

So this change restores SERVICE_TENANT_NAME, to be able to switch to SERVICE_PROJECT_NAME.

Related-bug: #1548634
Change-Id: I14ebf23aa63f0f153b934ad213a6209d22e73e9d
diff --git a/lib/keystone b/lib/keystone
index e33081c..ff9714f 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -108,6 +108,7 @@
 # Bind hosts
 KEYSTONE_ADMIN_BIND_HOST=${KEYSTONE_ADMIN_BIND_HOST:-$KEYSTONE_SERVICE_HOST}
 # Set the tenant for service accounts in Keystone
+SERVICE_TENANT_NAME=${SERVICE_TENANT_NAME:-service}
 SERVICE_PROJECT_NAME=${SERVICE_TENANT_NAME:-service}
 
 # if we are running with SSL use https protocols