Fix request timeout errors during calls to NSX controller

Sometimes two correlated exception traces are observed in
the server log for the Neutron Server backed by NSX:
RequestTimeout (The nsx request has timed out) and
OperationalError (Lock wait timeout exceeded). This is
generally described by Guru Salvatore Orlando as the,
and I quote, the "infamous eventlet-mysql deadlock".

This patch tries to address the issue by adding a
cooperative yield in the nsx client code (it’s a good idea
to call sleep(0) occasionally in any case) and also by
avoiding the unnecessary spawning of another Greenthread
within a call that is already executed in Greenthred
itself.

Closes-bug: #1267101
Related-bug: #1279497

Change-Id: I8e298468fb730f11a66fbd4211121ee7d3e2a548
BigSwitch: Move config and REST to diff modules

No functionality change. Separates the config,
rest call, and backend server management from
the main plugin.py file. Necessary to make
downstream patches more managable and easier
to review.

Implements: blueprint bigswitch-separate-server-module
Change-Id: Ie1fd18a9d8cde24945513c06f7b62239202258a3
Merge "BigSwitch: Move config and REST to diff modules"
Merge "Fix request timeout errors during calls to NSX controller"
Imported Translations from Transifex

Change-Id: Ia424e2ea79fa28a561b7d4a85178d111fd0a7f6e