Only map nicira_nvp_plugin module if installed
Only map nicira_nvp_plugin module if installed

When starting any quantum plugin agent service, only map the
nicira_nvp_plugin module if nicira is installed and configured for
quantum.

fixes bug 1175750

Change-Id: I7f2c40011363e2579d4c35716f5a7d413663a698

gkotton/vmware-nsx,gkotton/vmware-nsx