FSAL_GLUSTER: Updated README and added sample gluster.conf file.

Cleaned up FSAL_GLUSTER README file and added
gluster.conf with minimum set of parameters required to 'src/config_samples'.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>
Acked-by: Niels de Vos <ndevos@redhat.com>