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
Enables packetfilter ext in NEC plugin based on its driver config

packet filter extension in NEC plugin should be determined based on
a return value of filter_supported method in the backend driver.
Previously filter_supported was not called actually and packetfilter
extension was always enabled.

Fixes bug #1133100

Change-Id: I1334c7695e64bc942c0ecce2bd85e5a92e75b388
Merge "Enables packetfilter ext in NEC plugin based on its driver config"
Merge "Set default api_extensions_path for NEC plugin"