Implements sync mechanism between Neutron and Nuage VSD

This will sync resources between Neutron and VSD based on
the configuration parameters. "enable_sync" will enable/disable
the sync and "sync_interval" will control the time interval
between consecutive sync cycles.

DocImpact

Change-Id: I6730bf0166dfd1e35795b679293a2831b5afbc75
Implements: blueprint nuage-neutron-sync
Merge "Implements sync mechanism between Neutron and Nuage VSD"