refactor: extract weave docker image name

Thanks to @hesco for the suggestion.

weaver/weave

@@ -29,6 +29,7 @@ command_exists () {
 
 [ $# -gt 0 ] || usage
 
+IMAGE=zettio/weave
 BRIDGE=weave
 CONTAINER_IFNAME=ethwe
 MTU=65535
@@ -229,11 +230,11 @@ case "$COMMAND" in
         IPADDR=$1
         shift 1
         case $(docker inspect --format '{{ .State.Running }} {{ .Config.Image }}' weave 2>/dev/null) in
-            "true zettio/weave")
+            "true $IMAGE")
                 echo "Weave is already running." >&2
                 exit 1
                 ;;
-            "false zettio/weave")
+            "false $IMAGE")
                 docker rm weave >/dev/null
                 ;;
             true*)
@@ -255,7 +256,7 @@ case "$COMMAND" in
         # (eth0, wlan0, etc) so the name becomes stable across host
         # restarts.
         MACADDR=`cat /sys/class/net/$BRIDGE/address`
-        CONTAINER=$(docker run --privileged -d --name=weave -p 6783:6783/tcp -p 6783:6783/udp $WEAVE_DOCKER_ARGS zettio/weave -name $MACADDR "$@")
+        CONTAINER=$(docker run --privileged -d --name=weave -p 6783:6783/tcp -p 6783:6783/udp $WEAVE_DOCKER_ARGS $IMAGE -name $MACADDR "$@")
         with_container_netns attach $CONTAINER $IPADDR eth0
         echo $CONTAINER
         ;;
@@ -280,8 +281,8 @@ case "$COMMAND" in
     version)
         # Try the running container first; if no luck try the image
         if ! DOCKERIMAGE=$(docker inspect --format='{{ .Image }}' weave 2>/dev/null) ; then
-            if ! DOCKERIMAGE=$(docker inspect --format='{{ .Id }}' zettio/weave 2>/dev/null) ; then
-                echo "Unable to find zettio/weave image." >&2
+            if ! DOCKERIMAGE=$(docker inspect --format='{{ .Id }}' $IMAGE 2>/dev/null) ; then
+                echo "Unable to find $IMAGE image." >&2
                 exit 1
             fi
         fi