Catch NotImplementedError on Network Associate

When trying to associate/disassociate a network both for project/host
using neutron a 500 error is returned because the "associate" method is
not even defined in neutronv2 api.

The changes are:
   * Add missing associate method to neutronv2 api and raise
     NotImplementedError
   * Catch NotImplementedError on the API to return the proper error
     code on network associate/disassociate operations.
   * Catch NotImplementedError on os_network when associating a network

Closes-Bug: #1293539
Change-Id: I75b980500f0f171b065b7606baf201ab89848e92
Merge "Catch NotImplementedError on Network Associate"