Merge pull request #28 from powerist/dhcpmod

Warning message modification in inception_dhcp.py
Zookeeper failover:

Handle failover for packet_in event.
Whenever a packet_in event is received, do logging first:
create a path on Zookeeper: /failover/"$dpid,$in_port",
whose value is msg.data, namely the raw ethernet packet.

During the process of packet_in event, all write operations
to Zookeeper, are bundled as one transaction, which is committed
after all other operations are finished.
Merge pull request #31 from powerist/working

Zookeeper failover:
Fix coding style according to flake8
Merge pull request #32 from changbl/working

Fix coding style according to flake8
Failover for DHCP.
Merge pull request #33 from powerist/working

Failover for DHCP.
Two minor naming changes

(*) transaction => txn

(*) zk_data => data_string
Merge pull request #34 from changbl/working

Two minor naming changes
Eliminate ARP broadcast:

No ARP broadcast in the network at all.
The controller is supposed to collect
hosts' info through gratuitous ARP, when
hosts boot.
If an ARP request finds no matching entry
in the ip_to_mac dictionary, either the
requested host does not exist, or the
gratuitous arp has not arrived at the
controller. The requesting host should
try later.