This is caused by the same underlying bug that triggered https://bugzilla.redhat.com/show_bug.cgi?id=1561637 The main difference is in how it's manifested. This bug tracks what happens when you click 'validate', and the other tracks the outcome when trying to create a provider with bad credentials.
The fix is here: https://github.com/ManageIQ/manageiq-providers-openstack/pull/265
Verified with Version 5.10.0.2.20180626170006_40dc459 showing proper flash `Credential validation was not successful: Login failed due to a bad username or password. `
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2019:0212