Remove rootwrap from incubator

oslo.rootwrap now fully graduated to a standalone library, and all three
consuming projects (Nova, Cinder and Neutron) use the library as of the
Icehouse release. There is no need to maintain the frozen code copy in
oslo-incubator anymore.

Implements bp remove-rootwrap

Change-Id: I522bb982f51472a2902ee9b0b7ade1d18351962b
Merge "Remove rootwrap from incubator"