lib/quantum: refactor quantum plugins and third party

As quantum plugin support is coming like floodlight, nvp and nec,
it's worth while to refactor quantum plugin logic so that each plugin can
be modified/enhanced intervening with other quantum plugin.
And new plugin support can be added easily (hopefully) without modifying
core logic.

Change-Id: Ic5ab5b993272fdd3b4e779823323777a845ee681
diff --git a/lib/nova b/lib/nova
index f0456d6..7165ae2 100644
--- a/lib/nova
+++ b/lib/nova
@@ -229,7 +229,7 @@
             configure_baremetal_nova_dirs
         fi
 
-        if is_service_enabled quantum && is_quantum_ovs_base_plugin "$Q_PLUGIN" && ! sudo grep -q '^cgroup_device_acl' $QEMU_CONF ; then
+        if is_service_enabled quantum && is_quantum_ovs_base_plugin && ! sudo grep -q '^cgroup_device_acl' $QEMU_CONF ; then
             # Add /dev/net/tun to cgroup_device_acls, needed for type=ethernet interfaces
             cat <<EOF | sudo tee -a $QEMU_CONF
 cgroup_device_acl = [