NSX: Fix request_id in api_client to increment

Previously, the NSX request_id in the api_client would always be
0 because the current request id was stored in the class which is
always initialized on each request. This patch fixes that by storing
the request_id as a class variable.

Change-Id: I9ff4d3eb09899fe6d14b462dddf9daddbf77873b
Closes-bug: 1325100
(cherry picked from commit 7a12da7be0acd58b737aeeaa6a9ce27a848c62dc)