java: make CephMountTest use user.* xattr names

Changes to the xattr code in Ceph require
a few tweaks to existing test cases.
Specifically, there is now a ceph.file.layout
xattr by default and user defined xattrs
are prepended with "user."

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Noah Watkins <noahwatkins@gmail.com>