Use oslo.rootwrap library instead of local copy

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

Partially-implements blueprint use-common-code

Change-Id: I519e8dec24dc9c48243af41dc0f423c05a0d92e2
Merge "Use oslo.rootwrap library instead of local copy"