Allow customizing the list of monitored network interfaces

This is a simple way to customize the selection of network interfaces. If INTERFACE_NAMES_OVERRIDE is set in muninlite.conf, that list is used instead of auto-detection. As muninlite.conf
itself is also a script, it is even possible to write a custom command.
This feature is helpful in environments with virtual machines or containers, like docker or lxc where there are lots of br-*, veth*, lxcbr* etc interfaces where monitoring doesn't make much
sense. I didn't find a way to reliably filter physical interfaces.

muninlite.in

@@ -78,7 +78,11 @@ RES=""
 for PLUG in $PLUGINS; do
   case "$PLUG" in
     if_|if_err_)
-      interface_names=$(sed 's/^ *//; s/:.*$//; / /d; /^lo$/d' /proc/net/dev)
+      if [ -z "$INTERFACE_NAMES_OVERRIDE" ]; then
+        interface_names=$(sed 's/^ *//; s/:.*$//; / /d; /^lo$/d' /proc/net/dev)
+      else
+        interface_names="$INTERFACE_NAMES_OVERRIDE"
+      fi
       for INTER in $interface_names; do
         INTERRES=$(echo "$INTER" | sed -e 's/\./VLAN/' -e 's/\-/_/g')
         RES="$RES ${PLUG}${INTERRES}"