Migrate from rootwrap to privsep

[hongbin]

Privsep [1][2] is the new library for managing execution of privileged command. It is better to switch to this library for security concern. The fix will be something like: https://review.openstack.org/#/c/299406/

[1] https://github.com/openstack/oslo.privsep
[2] https://specs.openstack.org/openstack/oslo-specs/specs/liberty/privsep.html