There were duplicated methods doing almost the same in terms
of OVS/OVN compilation.
* 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
* Unify NEUTRON_PATH variable to NEUTRON_DIR
* Sets default variable for OVN_BRANCH to point latest
(cherry picked from commit fb2806f808)