rgw/COSBench : Cosbench to use details of existing user, access and secret keys.

[viduship]

Cosbench script to use details of an existing user, access, and secret keys.

The cosbench.py create a new rgw user. We would like to add the capability to use the existing user to run the workloads.
Also, since user creation is a metadata operation, it would, in turn, help in running the workloads from the secondary site in a multisite environment.