Handle more nicely when role root is already here

When using postgresql we were handling the fallback if the role root was
already here but this was still printing an error message, try to make
it a bit smarter.

Closes-Bug: #1265477
Change-Id: Ib3768dd182ab968e81038f900550f641b9a2af5c
Add oslo.rootwrap to devstack gate

oslo.rootwrap recently graduated but was not made part of the
devstack-gate. This change is part of a series of changes affecting
devstack-gate, config and devstack which will collectively fix this:

https://review.openstack.org/#/q/status:open+topic:rootwrap-gate,n,z

This should probably be merged once the config and devstack-gate changes
are in, so that it can be self-testing.

Change-Id: I7b1332c8004845a0dd76e27d871370d41d4524ac
Merge "Handle more nicely when role root is already here"
Merge "Add oslo.rootwrap to devstack gate"