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>
mds:  Add retry field to MDRequest

The MDRequest can be retried via the RetryRequest finisher,
we add a retry field that gets incremented before each
retry in the finisher.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
mds: Setting pool on a file requires latest osdmap

If we create a file, then create a pool, then try to
set the pool on the file with the vxattr, no mds operation
triggers a request for the latest osdmap, so we get back
EINVAL.  So if we fail to find the pool initially, fall back
to requesting a new osdmap and trying the vxattr again.  We
don't wait for a new map if we don't get anything newer than
what we already have.

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