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 126.96.36.199.20180626170006_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.