rbd.py: add xfstests functionality

rbd.py: add xfstests functionality

Add tasks for running xfstests over a pair of rbd volumes.  The main
one is called xfstests, and it sets up rbd volumes of specified size
and runs a set of likely-to-be-successful tests.  The other one is
used by the first, and is called run_xfstests.  This provides a
generic (device rather than rbd device oriented) interface to
xfstests, and should probably be made standalone and distinct from
rbd at some point.

Using multiple rbd devices required the rbd udev rule manipulation
to ignore errors, since it appears that each device caused the a
teardown attempt, which leads to failures the second time around.
There's probably a more robust solution, but this works for now.

Signed-off-by: Alex Elder <elder@dreamhost.com>