This patch moves ovs_lib from agent/linux to agent/common
since it will be used by ovs_neutron_agent on both Linux
and Windows platforms. To provide compatibility with out of
tree code, a placeholder has been left in agent/linux.
Unit tests are updated accordingly.
Partially implements blueprint: hyper-v-ovs-agent
Change-Id: I009f7f5e3b014633541ed5a45628aa1b2287e22b