After merge of change [1] to devstack project, preconfigured creds for
Tempest cannot be used anymore. But Manila requires such.
For the moment, latest Tempest interfaces cannot allow us to use
preconfigured creds because of bug #1524717.
So, until this bug fixed, we should workaround Devstack changes and
set legacy opts explicitly.
[1] I65b56ff681d6c27094380693c953fbc3664eceb0
Related-Bug: #1531049
Closes-Bug: #1531189
Change-Id: I4dc266713ec06bc6e98616a1277c8ae6cadf199d
84bb7226ce