Set default api_extensions_path for NEC plugin

NEC plugin has a plugin specific extension in its plugin directory.
api_extension_path must be set in quantum.conf to enable quantum-server
to load the extension, but users sometimes forget to set it. It provides
better usability if api_extensions_path is set appropriately by default.

Fixes bug 1133099

Change-Id: Ifa6d8a4f6bd30be5c6641c127ffcfaab5ff20b47
Merge "Set default api_extensions_path for NEC plugin"