rbd: allow client id to be specified in config string

Allow the client id to be specified in the config string via 'id=' so that
users can control who they authenticate as.  Currently they are stuck with
the default ('admin').  This is necessary for anyone using authentication
in their environment.

Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
