[mgcp] Make the number of endpoints configurable...

Allow to configure the number of endpoints at start. Currently
this will not be changed at runtime but one needs to save the
config and restart the system.
diff --git a/openbsc/src/mgcp.cfg b/openbsc/src/mgcp.cfg
index d9950ab..678f546 100644
--- a/openbsc/src/mgcp.cfg
+++ b/openbsc/src/mgcp.cfg
@@ -15,4 +15,5 @@
   rtp base 4000
   sdp audio payload number 98
   sdp audio payload name AMR/8000
+  number endpoints 31
   loop 1