interface-reconfigure.at: Use $PYTHON explicitly

Workaround pkg_alternative issue for NetBSD:
    http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=51152

An alternative would be generating xenserver scripts from *.in,
but i'm not sure if it's appropriate for those scripts.

Signed-off-by: YAMAMOTO Takashi <yamamoto@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

tests/interface-reconfigure.at

@@ -681,7 +681,7 @@ EOF
 }
 
 ifr_run () {
-    ./interface-reconfigure --root-prefix="`pwd`" --no-syslog "$@"
+    $PYTHON ./interface-reconfigure --root-prefix="`pwd`" --no-syslog "$@"
 }
 
 ifr_filter () {