l2_switch_v1_3.py: for match fields, use a list rather than a dict.

python dict, which is unordered, is not appropriate to represent
a list of ofp match field, which is ordered.

while this example currently does not seem to use order-sensitive
fields, copy-and-modified scripts can.
eg. http://thread.gmane.org/gmane.network.ryu.devel/3082