Allow a plugin specific interface_driver config in q-lbaas

Previously interface_driver for q-lbaas is hardcoded and q-lbaas does
not work when quantum plugin requires additional configuration (like
ovs_use_veth=True). This commit set up interface_driver using
quantum_plugin_setup_interface_driver function.

This commit also moves a script related Quantum service plugin to
lib/quantum_plugins/services/

Change-Id: Iad11c1b31071dfb580843be18be78c83feb91f10
diff --git a/lib/quantum b/lib/quantum
index efdd43d..b088202 100644
--- a/lib/quantum
+++ b/lib/quantum
@@ -179,7 +179,7 @@
 # Agent loadbalancer service plugin functions
 # -------------------------------------------
 # Hardcoding for 1 service plugin for now
-source $TOP_DIR/lib/quantum_plugins/agent_loadbalancer
+source $TOP_DIR/lib/quantum_plugins/services/agent_loadbalancer
 
 # Use security group or not
 if has_quantum_plugin_security_group; then