Reduce resource requirements for some test runs

Adds --instance_group_size parameter and skips creating IGs in the
secondary zone is not used by any of the specified test cases.

tools/run_tests/run_xds_tests.py

@@ -143,6 +143,15 @@ argp.add_argument('--source_image',
 argp.add_argument('--machine_type',
                   default='e2-standard-2',
                   help='Machine type for VMs created during the test')
+argp.add_argument(
+    '--instance_group_size',
+    default=2,
+    type=int,
+    help=
+    'Number of VMs to create per instance group. Certain test cases (e.g., '
+    'round_robin) may not give meaningful results if this is set to a value '
+    'less than 2.'
+)
 argp.add_argument(
     '--tolerate_gcp_errors',
     default=False,
@@ -163,7 +172,7 @@ if args.verbose:
 _DEFAULT_SERVICE_PORT = 80
 _WAIT_FOR_BACKEND_SEC = args.wait_for_backend_sec
 _WAIT_FOR_OPERATION_SEC = 300
-_INSTANCE_GROUP_SIZE = 2
+_INSTANCE_GROUP_SIZE = args.instance_group_size
 _NUM_TEST_RPCS = 10 * args.qps
 _WAIT_FOR_STATS_SEC = 180
 _WAIT_FOR_URL_MAP_PATCH_SEC = 300
@@ -188,6 +197,12 @@ _BOOTSTRAP_TEMPLATE = """
     ]
   }}]
 }}""" % (args.network.split('/')[-1], args.zone, args.xds_server)
+_TESTS_USING_SECONDARY_IG = [
+    'secondary_locality_gets_no_requests_on_partial_primary_failure',
+    'secondary_locality_gets_requests_on_primary_failure'
+]
+_USE_SECONDARY_IG = any(
+    [t in args.test_cases for t in _TESTS_USING_SECONDARY_IG])
 _PATH_MATCHER_NAME = 'path-matcher'
 _BASE_TEMPLATE_NAME = 'test-template'
 _BASE_INSTANCE_GROUP_NAME = 'test-ig'
@@ -937,6 +952,7 @@ try:
     template_name = _BASE_TARGET_PROXY_NAME + args.gcp_suffix
     instance_group_name = _BASE_INSTANCE_GROUP_NAME + args.gcp_suffix
     same_zone_instance_group_name = _BASE_INSTANCE_GROUP_NAME + '-same-zone' + args.gcp_suffix
+    if _USE_SECONDARY_IG:
         secondary_zone_instance_group_name = _BASE_INSTANCE_GROUP_NAME + '-secondary-zone' + args.gcp_suffix
     try:
         create_health_check(gcp, health_check_name)
@@ -977,6 +993,7 @@ try:
         patch_backend_instances(gcp, backend_service, [instance_group])
         same_zone_instance_group = add_instance_group(
             gcp, args.zone, same_zone_instance_group_name, _INSTANCE_GROUP_SIZE)
+        if _USE_SECONDARY_IG:
             secondary_zone_instance_group = add_instance_group(
                 gcp, args.secondary_zone, secondary_zone_instance_group_name,
                 _INSTANCE_GROUP_SIZE)
@@ -1020,10 +1037,12 @@ try:
                     same_zone_instance_group_name, result['selfLink'],
                     args.zone)
                 gcp.instance_groups.append(same_zone_instance_group)
+                if _USE_SECONDARY_IG:
                     result = compute.instanceGroups().get(
                         project=args.project_id,
                         zone=args.secondary_zone,
-                    instanceGroup=secondary_zone_instance_group_name).execute()
+                        instanceGroup=secondary_zone_instance_group_name
+                    ).execute()
                     secondary_zone_instance_group = InstanceGroup(
                         secondary_zone_instance_group_name, result['selfLink'],
                         args.secondary_zone)