packet lib: improve readability of json

this patch makes some json members of several packet libraries human-readable.

ex) ethernet.ethernet

before applying this patch:

    {'ethernet': {'ethertype': 2048, 'dst': 'ZmY6ZmY6ZmY6ZmY6ZmY6ZmY=', 'src': 'MDA6MDA6MDA6MDA6MDA6MDA='}}

after applying this patch:

    {'ethernet': {'ethertype': 2048, 'dst': u'ff:ff:ff:ff:ff:ff', 'src': u'00:00:00:00:00:00'}}

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
lib: ofctl: support the specified max_len of flow_mod

this patch makes the max_len of flow_mod message more flexible.

before applying this patch:

    ofctl_v1_0: 128 bytes for packet_in (fixed size) and none for others (fixed)
    ofctl_v1_2: 128 bytes for packet_in (fixed size) and 0 for others (fixed size)
    ofctl_v1_3: 0 for all by default (flexible)

    ofctl_v1_3 will send 0 byte length data in a packet_in message if max_len is not specified.

after applying this patch:

    all version: MAX_LEN for all by default (flexible)
    NOTE: MAX_LEN is 65535 for OF1.0, and is OFPCML_MAX for OF1.2/1.3.

    max_len is omissible by every ofctl.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
rest_firewall: use the explicit max_len of flow_mod

rest_firewall had used the max_len value of flow_mod message which ofctl_v1_x specified.
in response to change ofctl_v1_x, rest_firewall gets to use the specified max_len explicitly.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>