Implement bind mount based provisioner backend for AppC.

This is a specialized backend that may be useful for deployments using large (multi-GB) single-layer images and where more recent kernel features such as overlayfs are not available.

For small images (10's to 100's of MB) the Copy backend may be sufficient.

1) It supports only a single layer. Multi-layer images will fail to provision and the container will fail to launch!

2) The filesystem is read-only because all containers using this image share the source. Select writable areas can be achieved by mounting read-write volumes to places like /tmp, /var/tmp, /home, etc. using the ContainerInfo. These can be relative to the executor work directory.

3) It relies on the image persisting in the store.

4) It's fast because the bind mount requires (nearly) zero IO.