Allow nicira plugin to handle multiple NVP API versions

Bug #1126352

This patch introduces a simple framework for enabling
nvlib to call the appropriate routine according to the
current version. To this aim, we leverage the 'server' header
which is returned by every NVP API calls (except login/logout).
The patch also accounts for the changes introduced in NVP 3.0

Change-Id: Ief3a6f2b5b99ea33548353ce3bee3fa23e42752f
Merge "Allow nicira plugin to handle multiple NVP API versions"