Adds nova-idmapshift cli utility

nova-idmapshift will be used by the libvirt-lxc driver to
correctly shift ownership of instance rootfs for use with
user namespaces.

Partially implements: bp libvirt-lxc-user-namespaces

Change-Id: I9aee622cca4578149201f02d02231a7bd0cbe912
Merge "Adds nova-idmapshift cli utility"