ceph_test_rados: add --pool <name> arg

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit bcfbd0a3ffae6947464d930f636c8b35d1331e9d)
ceph_test_rados: print version banner on startup

It is helpful when looking at qa run logs to see what version of the
tester is running.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 12c1f1157c7b9513a3d9f716a8ec62fce00d28f5)