There were duplicated methods doing almost the same in terms
of OVS/OVN compilation.
This change:
* move of OVS related compilation code to devstack/lib/ovs
* delete of OVS related compilation code from devstack/lib/ovn_agent
* source unified functions in devstack/lib/ovn_agent from
devstack/lib/ovs
* Unify NEUTRON_PATH variable to NEUTRON_DIR
Closes-Bug: #1877377
Change-Id: Ia012a8e116a276a6674f86366c803e0e2d8ff704