Add documentation for setup ceph client authentication

[timureh]

There is an option cluster_client_name in the iscsi-gateway.cfg config. I don't have the opportunity to use the client.admin user. Could you describe precisley in the documentation the minimal set of capabilities required for work ceph-iscsi without access denied. For example like this.
I did it like this:

ceph auth get-or-create client.iscsi mon 'profile rbd, allow command "osd blacklist", allow command "osd blocklist"' osd 'profile rbd'