New info on user roles, swauth tools and authentication

Adding new information to the authentication guide on how user
roles are defined in gswauth. Also provided information on all
gswauth tools and how to authenticate as a user.

Change-Id: If565f4bfa9d112cdf7778f930cb4f75ee670d6c4
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6494
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>