Closes #1813

This adds preliminary (but working) support for creating application
containers from OCI formats.  Examples:

create a container from a local OCI layout in ../oci:

    sudo lxc-create -t oci -n a1 -- -u oci:../oci:alpine

Or, create a container pulling from the docker hub.

    sudo lxc-create -t oci -n u1 -- -u docker://ubuntu

The url is specified in the same format as for 'skopeo copy'.

Comments appreciated.
Signed-off-by: Serge Hallyn <shallyn@cisco.com>