Fix request timeout errors during calls to NSX controller

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