add VTY commands: mncc internal / external (== -M)

So far the only way to use external MNCC is to pass the -M cmdline arg:

  osmo-msc -M /path/to/socket

However, the osmo-msc.service file for systemd is installed by 'make install',
and hence it is quite impractical to depend on such a config item to be
required in the service file:

- It defies any scheme an operator may have in place to compose the
  osmo-msc.cfg file -- this option doesn't go in the .cfg file but needs
  separate action to add to the installed service file.

- After a make install or package upgrades / re-installations, this option will
  be plain overwritten silently, or lead to the need for resolving file
  conflicts.

The initial spark for this came from configuring the 35c3 GSM from cfg
templates.

Change-Id: I2ec59d5eba407f83295528b51b93678d446b9cee
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index 9fa9d2a..c4ab387 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -30,6 +30,8 @@
 OsmoMSC(config-msc)# list
 ...
   assign-tmsi
+  mncc internal
+  mncc external MNCC_SOCKET_PATH
   mncc guard-timeout <0-255>
   no assign-tmsi
   auth-tuple-max-reuse-count <-1-2147483647>
@@ -46,6 +48,29 @@
   mgw bts-base <0-65534>
 ...
 
+OsmoMSC(config-msc)# mncc?
+  mncc  Configure Mobile Network Call Control
+
+OsmoMSC(config-msc)# mncc ?
+  internal       Use internal MNCC handler (default; changes need a program restart)
+  external       Use external MNCC handler (changes need a program restart)
+  guard-timeout  Set global guard timer for mncc interface activity
+
+OsmoMSC(config-msc)# mncc external ?
+  MNCC_SOCKET_PATH  File system path to create the MNCC unix domain socket at
+
+OsmoMSC(config-msc)# mncc external /path/not/used
+OsmoMSC(config-msc)# show running-config
+...
+msc
+...
+ mncc external /path/not/used
+...
+
+OsmoMSC(config-msc)# mncc internal
+OsmoMSC(config-msc)# show running-config
+... ! mncc external
+
 OsmoMSC(config-msc)# exit
 OsmoMSC(config)# mncc-int
 OsmoMSC(config-mncc-int)# list