Use oslo.rootwrap library instead of local copy

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

Patch based on: https://review.openstack.org/#/c/67422

Change-Id: I3dd2c4cab6652dea5bd755b6048cbeea9edc81cf
Partial-Bug: #1268674
Merge "Use oslo.rootwrap library instead of local copy"