Moves non-configurable variables to "constants" section
Moves non-configurable variables to "constants" section

Moves the non caonfigurable settings OVS_DB_CONF_DIR and
OVS_DB_SOCKET_DIR to their own section for clarity

All configurable settings are documented in doc/source/usage.rst

Closes-Bug #1513834

Change-Id: I36cf5acd2d166e3ebcfb276d595e83bef3be1b2e

openstack/networking-ovs-dpdk,openstack/networking-ovs-dpdk,stackforge/networking-ovs-dpdk,stackforge/networking-ovs-dpdk,stackforge/networking-ovs-dpdk