Adds some utility methods and a couple of base test cases that
can be added to. These first tests exercise the ovs driver (dnsmasq)
and so the code is organised accordingly - OVS specific test cases
are defined in a DHCPAgentOVSTestFramework
Partial-Bug: #1469065
Co-Authored-By: Cedric Brandily <zzelle@gmail.com>
Co-Authored-By: Sergey Belous <sbelous@mirantis.com>
Change-Id: Ic9d5a2f2b8014e4d81f5e5f6fa58b119a86de075