Allow tests from test_dhcp_agent run independently

Some tests from test_dhcp_agent depend on each other
and cannot be run in arbitrary order. TestDhcpAgent
and TestDhcpAgentEventHandler need config options that
are initialized in other tests. This patch eliminates
such dependencies.

Fixes: bug #1125951
Change-Id: I10bc46a5e1244ea0f7a05a20bb162c50303f282e
Merge "Allow tests from test_dhcp_agent run independently"