Pick a random zone for CLI tests. (#2004)

This changes the CLI end-to-end test so that it defaults to
picking a random zone in the region 'us-west' instead of being
hard-coded to 'us-west1-a'.

The test will still honor the 'zone' config setting in gcloud if
it is set; this only changes the behavior when that is not set.

tools/cli/tests/end-to-end.py

@@ -18,6 +18,7 @@
 # of the bundled CLI tool. This requires a GCP project in which the
 # test will create, connect to, and delete Datalab instances.
 
+import random
 import socket
 import subprocess
 import sys
@@ -58,6 +59,15 @@ def free_port():
     return port_number
 
 
+def random_zone():
+    zones_list = subprocess.check_output([
+        'gcloud', 'compute', 'zones', 'list',
+        '--filter=region~us-west', '--format=value(name)']).decode(
+            'utf-8')
+    zones = zones_list.split()
+    return random.choice(zones)
+
+
 class DatalabInstance(object):
     def __init__(self, test_run_id, project, zone):
         self.project = project
@@ -154,7 +164,7 @@ def setUp(self):
         self.zone = call_gcloud(
             ['config', 'get-value', 'compute/zone']).strip()
         if self.zone == '':
-            self.zone = 'us-west1-a'
+            self.zone = random_zone()
         print('Testing with in the zone {} under the project {}'.format(
             self.zone, self.project))