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
3 files changed