Enhancements and fixes to various agents

* bugfixes to l3, dhcp, and openvswitch
* plugin lookup fixes for spec tests
* install kernel headers for openvswitch
* add spec tests for openvswitch
* add spec tests for dhcp_agent
* add spec skeleton tests for l3_agent

Change-Id: I68177c191e355dbfced23834a627cca5adf7abee