[type/__interface] Add --onchange parameter

cdist/conf/type/__interface/files/backends/ifupdown.d/manifest

@@ -92,6 +92,11 @@ then
 	set -- "$@" --comment "$(cat "${__object}/parameter/comment")"
 fi
 
+if test -s "${__object}/parameter/onchange"
+then
+	set -- "$@" --onchange "$(cat "${__object}/parameter/onchange")"
+fi
+
 if test -s "${__object}/parameter/extra-config"
 then
 	while read -r _opt

cdist/conf/type/__interface/man.rst

@@ -57,6 +57,17 @@ extra-config
     Using this option makes the configuration non-portable to other backends.
     If this option is used extensively, it is recommended to use the respective
     backend type directly.
+onchange
+    The action to perform if the interface configuration has changed.
+    Available options are:
+    leave (default)
+      Do nothing.
+    up
+      Bring the interface up if it is down.
+    down
+      Bring the interface down if it is up.
+    refresh
+      Refresh the interface (down && up).
 
 
 BOOLEAN PARAMETERS

cdist/conf/type/__interface/parameter/default/onchange

@@ -0,0 +1 @@
+leave

cdist/conf/type/__interface/parameter/optional

@@ -3,5 +3,6 @@ comment
 gateway
 name
 netmask
+onchange
 state
 type