fix documentation examples for with_nested

In the mailing list, Edgars Mazurs commented that the docs' example for with_nested could be wrong: https://groups.google.com/forum/#!topic/ansible-project/PpRrBQv_T9s

This made me test the examples, and it turns out the example itself was almost good: there was an sql-related bug and I had missed the required password parameter. I've fixed the errors and tested the new examples before pushing. Sorry for that. From now on, I'll test the actual examples I add to the docs before pushing.

GATHERING FACTS ***************************************************************
ok: [testhost]

TASK: [give users access to multiple databases] *******************************
changed: [testhost] => (item=['alice', 'clientdb'])
changed: [testhost] => (item=['alice', 'employeedb'])
changed: [testhost] => (item=['alice', 'providerdb'])
changed: [testhost] => (item=['bob', 'clientdb'])
changed: [testhost] => (item=['bob', 'employeedb'])
changed: [testhost] => (item=['bob', 'providerdb'])
changed: [testhost] => (item=['eve', 'clientdb'])
changed: [testhost] => (item=['eve', 'employeedb'])
changed: [testhost] => (item=['eve', 'providerdb'])

PLAY RECAP ********************************************************************
testhost : ok=2    changed=1    unreachable=0    failed=0