don't fail on missing docker_tls_args

instead just don't auto-detect TLS args in that case.

This makes `weave --local launch` work (again).

Fixes #1844.

weave

@@ -1455,7 +1455,7 @@ proxy_args() {
     PROXY_HOST=""
     proxy_parse_args "$@"
 
-    if [ -z "$PROXY_TLS_ENABLED" -a -z "$PROXY_TLS_DETECTION_DISABLED" ] ; then
+    if [ -z "$PROXY_TLS_ENABLED" -a -z "$PROXY_TLS_DETECTION_DISABLED" ] && command_exists docker_tls_args ; then
         if ! DOCKER_TLS_ARGS=$(docker_tls_args) ; then
             echo "Unable to auto-detect proxy TLS configuration; you must launch the proxy with" >&2
             echo "'weave launch-proxy' and supply '--no-detect-tls' or TLS options." >&2