Fix incorrect test for list_users

The current test would not fail if incorrect data was returned by
the driver.  The list_roles test also has the same issue.

Fixes bug 1222574

Change-Id: Icc5e76e9453436ac208c5f262d0dd4082b700d02
domain-specific drivers experimental in havana

Related-Bug: 1218094
Related-Bug: 1217017
Change-Id: I5fecaa8c6531b2b38c914908ecff8a33de67857c
Merge "domain-specific drivers experimental in havana"
Merge "Fix incorrect test for list_users"