test: Add optional timestamps to ceph_test_rados

With this commit, if the --timestamps parameter is passed to
ceph_test_rados, most output lines are prefixed with a timestamp.

If not specified, the output should be identical to before.

This is useful for correlating what the test is doing with  ceph
osd logs.

Signed-off-by: Alex Ainscow <[email protected]>

Author: aainscow