Make rados/rest bench work for multiple write instances without metadata conflict.

rados bench is useful to benchmark the cluster, however, there is one user case it does not support: read after write if there are multiple write instance, this is because the current implementation hard code the metadata object which will make the last update win, as a result, the metadata of the previous run is lost and user is unable to run a read (seq) based on that round of write.

This patch provides user the ability to specify a run-name where to store the metadata (a default one is used if user does not provide one).

Signed-off-by: Guang Yang (yguang@yahoo-inc.com)