Provide postinst script, taking care of services restarts in HA mode

Addressing https://bugtracker.sipwise.com/view.php?id=1225

From: Michael Prokop <mprokop@sipwise.com>

debian/ngcp-mediator.postinst

@@ -0,0 +1,54 @@
+#!/bin/sh
+# postinst script for ngcp-mediator
+
+set -e
+
+restart_handler() {
+  if [ -x "/etc/init.d/mediator" ]; then
+    if [ -x "$(which invoke-rc.d 2>/dev/null)" ]; then
+      invoke-rc.d mediator restart || exit $?
+    else
+      /etc/init.d/mediator restart || exit $?
+    fi
+  fi
+}
+
+initscript_handler() {
+  if [ -x "/etc/init.d/mediator" ]; then
+    update-rc.d mediator defaults >/dev/null
+    invoke-rc.d mediator start || exit $?
+  fi
+}
+
+init_handler() {
+  # just invoke init script wrappers on ce systems since
+  # they do not provide ngcp-check_active and we don't
+  # have to handle inactive nodes
+  if ! [ -x "$(which ngcp-check_active 2>/dev/null)" ]; then
+    restart_handler
+    initscript_handler
+  else # do not restart daemon on inactive node in pro systems
+    if ngcp-check_active ; then
+      echo "Active node detected, restarting mediator"
+      restart_handler
+    else
+      echo "Inactive node detected, ignoring request to restart mediator"
+    fi
+  fi
+}
+
+case "$1" in
+    configure)
+      init_handler
+    ;;
+
+    abort-upgrade|abort-remove|abort-deconfigure)
+    ;;
+
+    *)
+        echo "postinst called with unknown argument \`$1'" >&2
+        exit 1
+    ;;
+esac
+
+exit 0