cephtool:  Add add_data_pool command to help

Signed-off-by: Sam Lang <sam.lang@inktank.com>
qa/workunits:  Add vxattr set pool test script

Add a test script that tests for creating a pool
and then setting the layout for a (pre-existing)
file to that pool.

Signed-off-by: Sam Lang <sam.lang@inktank.com>