Add DNS and DHCP log into dhcp agent

Add DNS and DHCP log into dhcp agent

Enable set DNS and DHCP log of dnsmasq for dhcp agent
Add a new configuration named 'dnsmasq_base_log_dir'
in dhcp_agent.ini.

This entry should be a path of log file. It should
like this:

    dnsmasq_base_log_dir=/tmp

And the DNS and DHCP log will be written into the file
"/tmp/aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa/dhcp_dns_log".
The dir path will be created if the given path doesn't exists.

DocImpact

Closes-Bug: #1475636
Change-Id: I87be346ec5059eaa8a29f48fe53933af82d1b155