Fix the argument check for account-generator

Concurrency parameter for account-generator command was accepting
negative values and zero.

This patch makes sure that when the account-generator is passed
a negative value or zero, it ends with an error.

Closes-Bug: #1811349
Partially-Implements: blueprint tempest-cli-unit-test-coverage
Change-Id: I4d7de89b2ad3ee91d113da3746fe393d8cce2aa2
diff --git a/releasenotes/notes/account-generator-accepts-positive-numbers-only-33a366a297494ef7.yaml b/releasenotes/notes/account-generator-accepts-positive-numbers-only-33a366a297494ef7.yaml
new file mode 100644
index 0000000..dfee1db
--- /dev/null
+++ b/releasenotes/notes/account-generator-accepts-positive-numbers-only-33a366a297494ef7.yaml
@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+   Concurrency parameter for account-generator command was accepting
+   negative values and zero. The concurrency parameter now accepts only
+   positive numbers. When a negative value or zero is passed to the
+   program then the program ends and help is displayed.