Fix KeystoneClient issue when token was expired

KeystoneClient should always first try to use username/password and only if
they are not provided, do a fallback to this.token. Otherwise for expired
tokens login is not possible.

Related-Bug: #1375622
(cherry picked from commit d8d9d96608f759d765ec0b1016e7363229ffc35a)

Change-Id: Ia85278402799d7d19e0c0aee975cc986de5879f9
Merge "Fix KeystoneClient issue when token was expired" into stable/5.1