Refactor dhcp checker to use pcap

Scapy packet filtering is slow and blocking.
Pcap filtering can be used in same thread,
this allows us to use higher timeout for dhcp offer waiting

Setup proper logger for dhcp_checker app.
ERROR level logs will be redirected to stderr
DEBUG level to /var/log/dhcp_checker.log

Change-Id: I7d9a0f1b8e3082637bc2474302fc0f7c17e0adb8
Closes-Bug: #1247284
Merge "Refactor dhcp checker to use pcap"