Fix alias for ext-gw-mode extension in nicira plugin

Also add a unit test for verifying that a 400 is raised when
enable_snat is an invalid string.

Bug 1207556

Change-Id: If33b8c72a08fbf7de1a837a65f083f09903c1809
Merge "Fix alias for ext-gw-mode extension in nicira plugin"
Imported Translations from Transifex

Change-Id: Ib406019d5ccc27576a4b01dd2566e2988cb82306