Make --concurrency not required in account_generator cmd

--concurrency arg in account_generator cmd is required
with default value 1.

This should be non required and work with default value if not given.

Change-Id: I105bd4833e5a0da58d900493a1782943c5506ffa
diff --git a/tempest/cmd/account_generator.py b/tempest/cmd/account_generator.py
index 0a5a41b..db323de 100755
--- a/tempest/cmd/account_generator.py
+++ b/tempest/cmd/account_generator.py
@@ -234,7 +234,7 @@
     parser.add_argument('-r', '--concurrency',
                         default=1,
                         type=int,
-                        required=True,
+                        required=False,
                         dest='concurrency',
                         help='Concurrency count')
     parser.add_argument('--with-admin',