postinst script: use invoke-rc.d instead of directly invoking init script

When invoking an init script directly we don't get the benefits
of what invoke-rc.d and its policy-rc.d framework gives us, so
instead use invoke-rc.d.

This fixes the lintian error:

| ngcp-www-admin: maintainer-script-calls-init-script-directly postinst:34

From: Michael Prokop <mprokop@sipwise.com>

debian/ngcp-www-admin.postinst

@@ -31,6 +31,8 @@ a2dissite default
 #DEBHELPER#
 
 # reload perl and/or apache modules
-/etc/init.d/apache2 reload
+if [ -x /etc/init.d/apache2 ] ; then
+  invoke-rc.d apache2 reload
+fi
 
 exit 0