Fix restarts of systemctl and dokku-installer

We previously assumed that systemd was detectable by the existence of certain directories. Unfortunately, it seems 14.04 already has these directories, so we need to handle this case separately.

One other note is that dokku-installer will fail to "start" if already started on certain systems, thus the previous grep was more accurate in it's running.

debian/postinst

@@ -56,8 +56,13 @@ case "$1" in
       /usr/share/dokku/contrib/dokku-installer.py onboot
 
       echo "Starting dokku-installer"
-      [[ -f /etc/systemd/system ]] && systemctl daemon-reload
-      service dokku-installer start
+      if command -v systemctl &>/dev/null; then
+        systemctl daemon-reload
+      fi
+
+      if service dokku-installer status 2> /dev/null | grep -E "waiting|inactive" ; then
+        service dokku-installer start
+      fi
     fi
 
     db_get "dokku/web_config"