rgw: add a load generation frontend

Still missing some pieces, but already generates requests.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: loadgen frontend read uid, init access key

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: sign loadgen requests

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: loadgen shutdown, error out on failures

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: loadgen, configurable num of objs, buckets

also fix request draining

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>