rpc/auth: allow SSL identity to be used for authorization

Access to a volume is now controlled by the following options, based on
whether SSL is enabled or not.

 * server.ssl-allow: get identity from certificate, no password needed

 * auth.allow: get identity and matching password from command line

It is not possible to allow both simultaneously, since the connection
itself is either using SSL or it isn't.

Change-Id: I5a5be66520f56778563d62f4b3ab35c66cc41ac0
BUG: 1114604
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/3695
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>