rbd: move xfstest TESTDIR=... bit to front of command

2013-09-24T11:32:49.169 DEBUG:teuthology.orchestra.run:Running [10.214.132.4]: "/usr/bin/sudo adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage TESTDIR=/home/ubuntu/cephtest /bin/bash /home/ubuntu/cephtest/run_xfstests.sh -c 1 -f xfs -t /dev/rbd1 -s /dev/rbd2 '1-9 11-15 17 19-21 26-29 31-34 41 46-
54 56 61 63-67 69-70 74-76 78-79 84-89 91'"
2013-09-24T11:32:49.211 INFO:teuthology.task.rbd.client.1.err:[10.214.131.22]: /usr/bin/ceph-coverage: 25: exec: TESTDIR=/home/ubuntu/cephtest: not found

Signed-off-by: Sage Weil <sage@inktank.com>