NSX plugin: return 400 for invalid gw certificate

Gateway certificates are validated by the NSX backend.
The code currently treats a failure in certification
validation as a backend failure and therefore returns
a 500 status code.

This patch changes this behaviour by returning a 400
status code and an appropriate error description.
To this aim a handler for 400 errors has been added to
the NSX API client.

Closes-Bug: #1293508

Change-Id: I196f14337e47cd40710a6d8a30bbe1cac5ffe05b
Merge "NSX plugin: return 400 for invalid gw certificate"