Set default value of --concurrency to 2

There is a non-negligible number of tests in tempest that need accounts
generated by account-generator with a concurrency of at least 2. Running
these tests with accounts generated with the current default value for
concurrency makes them therefore fail.

This change sets the default value for concurrency to 2 and updates
the documentation for --concurrency parameter.

Closes-Bug: #1948935
Change-Id: Iada980beff9db3c8cd02c84d0d7602264b66ddf8

releasenotes/notes/set-default-value-of-concurrency-to-2-d916d5c31e3725d5.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    [`bug 1948935 <https://bugs.launchpad.net/tempest/+bug/1948935>`_]
+    The default value of account-generator --concurrency parameter is now
+    set to 2 instead of 1.

tempest/cmd/account_generator.py

@@ -81,11 +81,11 @@ Optional Arguments
   will have the prefix with the given TAG in its name. Using tag is recommended
   for the further using, cleaning resources.
 
-* ``-r, --concurrency CONCURRENCY`` (Optional) Concurrency count
-  (default: 1). The number of accounts required can be estimated as
-  CONCURRENCY x 2. Each user provided in *accounts.yaml* file will be in
-  a different tenant. This is required to provide isolation between test for
-  running in parallel.
+* ``-r, --concurrency CONCURRENCY`` (Optional) Concurrency count (default: 2).
+  The number of accounts generated will be same as CONCURRENCY. The higher the
+  number, the more tests will run in parallel. If you want to run tests
+  sequentially then use 1 as value for concurrency (beware that tests that need
+  more credentials will fail).
 
 * ``--with-admin`` (Optional) Creates admin for each concurrent group
   (default: False).
@@ -236,7 +236,7 @@ def _parser_add_args(parser):
                         dest='tag',
                         help='Resources tag')
     parser.add_argument('-r', '--concurrency',
-                        default=1,
+                        default=2,
                         type=positive_int,
                         required=False,
                         dest='concurrency',