Use oslo.rootwrap library instead of local copy

Remove rootwrap code copied from oslo-incubator, make
cinder-rootwrap a console_script entrypoint pointing in
oslo.rootwrap instead.

Change-Id: I4e924bc9fccd7f5a8522ac38b7c6d29683990ea8
Implements: blueprint cinder-oslo-rootwrap
Merge "Use oslo.rootwrap library instead of local copy"