Allow tests in TestDhcpAgentEventHandler run independently

Tests in TestDhcpAgentEventHandler depend on dhcp_confs
option that is defined in quantum.agent.linux.dhcp. We have
to register the option in order to allow independent testing.

This problem is similar to bug #1125951.

Fixes: bug #1156696

Change-Id: I323908c6f70697e056294b1817ff32ce5d32b32c