iscsi-target: Add demo-mode TPG authentication context support

This patch adds a auth configfs group context following existing
explict NodeACL and discovery auth within:

  /sys/kernel/config/target/iscsi/$TARGETNAME/$TPGT/auth/

This patch allows these attributes to be used for CHAP authentication
an TPG is configured in demo-mode (generate_node_acl=1).

Note this authentication information takes precedence over NodeACL
authentication when struct se_node_acl->dynamic_node_acl is present.

Cc: Dax Kelson <dkelson@gurulabs.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>