Nicira NVP plugin support for l3_ext_gw_mode extension

Bug 1121129

This patch adds support the 'configurable external gateway' extension
in the NVP plugin.

Change-Id: I531ebe0053b1b9e21d6f0685776acebe3173b170
Merge "Nicira NVP plugin support for l3_ext_gw_mode extension"