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 "Add oslo.rootwrap to devstack gate"