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
Removing vim header lines

Simply removes the vim header line and the empty line
following it (when present) for files in the api, db, and
extenssions base directories. In one single _init_ file in
this patchset, all comment lines are removed.

Change-Id: I81b7168dc2ad74b8c30a87e50a531e157851499e
Partial-Bug: 1229324
Merge "NSX plugin: return 400 for invalid gw certificate"
Merge "Removing vim header lines"