ceph: change ceph.file.layout.* implementation, content

Implement a new method to generate the ceph.file.layout vxattr using
the new framework.

Use 'stripe_unit' instead of 'chunk_size'.

Include pool name, either as a string or as an integer.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>