Add check if user shell exists, and verify the shell is allowed on AIX

[scotthain]

Signed-off-by: Scott Hain shain@chef.io

Description

[Describe what this change achieves]

Issues Resolved

[List any existing issues this PR resolves]

Check List

• [ ] All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• [ ] New functionality includes testing.
• [ ] New functionality has been documented in the README if applicable
• [ ] All commits have been signed for the Developer Certificate of Origin. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/CONTRIBUTING.MD

[scotthain]

@chef-cookbooks/engineering-services

[scotthain]

@schisamo @jeremiahsnapp I've incorporated both your feedback - thanks!