Provide a default api_extensions_path for nvp_plugin

The NVP plugin has several extensions within it's plugin directory. In order
to tell quantum-server to load this, api_extensions_path must be set in
quantum.conf. This is sort of painful because one needs to set this value.

Fixes bug 1131469

Change-Id: I8d124aa93f79dbeb8b2abac1c27765d5ebf449c2
Merge "Provide a default api_extensions_path for nvp_plugin"