qa: cleanup cephtool/test.sh tmp files

When run in a shared environment ( as opposed as a machine created for
the purpose of running this test only ), it is important to cleanup
leftovers to avoid poluting the /tmp space. Create a common temporary
directory for all tmp files.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1084 from dachary/wip-cephtool-test

qa: cleanup cephtool/test.sh tmp files

Reviewed-by: Sage Weil <sage@inktank.com>