Use oslo.rootwrap library instead of local copy

Remove rootwrap code copied from oslo-incubator, make the
nova-rootwrap console_script entrypoint point to oslo.rootwrap
code instead.

Change-Id: Iec7aaf2e1599b2faf403d11b779eeab199a89486
Implements: blueprint nova-oslo-rootwrap
Merge "Use oslo.rootwrap library instead of local copy"