[ansible/artifactory] Assert that database username and password are defined.

[bbaassssiiee]

PR Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [X] Title of the PR starts with installer/product name (e.g. [ansible/artifactory])
• [ ] CHANGELOG.md updated
• [ ] Variables and other changes are documented in the README.md

What this PR does / why we need it: The looping in the creation tasks for user, database and privileges in the postgres role did not fail on undefined values. Input validation was missing, and the logging of item=None is not so useful.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #311

Special notes for your reviewer:

[Logeshwarsn]

@bbaassssiiee Could you please address the comment ?