osmo-sgsn: add VTY parameter to toggle authentication

It may be useful to have 'remote' authorization policy, but do not
require authentication in GERAN at the same time, e.g. in combination
with 'subscriber-create-on-demand' feature of OsmoHLR.

This change introduces a new VTY parameter similar to the one
that we already have in OsmoMSC:

  authentication (optional|required)

Please note that 'required' only applies if 'auth-policy' is 'remote'.

Change-Id: I9909145e7e0af587c28827e16301a61b13eedaa9
diff --git a/doc/examples/osmo-sgsn/osmo-sgsn-accept-all.cfg b/doc/examples/osmo-sgsn/osmo-sgsn-accept-all.cfg
index b47878a..85112f4 100644
--- a/doc/examples/osmo-sgsn/osmo-sgsn-accept-all.cfg
+++ b/doc/examples/osmo-sgsn/osmo-sgsn-accept-all.cfg
@@ -10,6 +10,7 @@
  ggsn 0 remote-ip 127.0.0.2
  ggsn 0 gtp-version 1
  ggsn 0 echo-interval 60
+ authentication optional
  auth-policy accept-all
 !
 ns