Introduce a timeout to radosbench's join phase

Set to 15min right now.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Use a timeout of config.get('time') * 2

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Merge pull request #218 from ceph/wip-radosbench-timeout

Introduce a timeout to radosbench's join phase