Neutron: Atomic update of instance info cache

In the Neutron network API implementation, there is a race condition
between a thread performing periodic task to read and heal instance
network info cache and another thread servicing interface-attach or
interface-detach calls. This patch ensures that instance info cache is
read and then updated in a synchronized block to ensure atomicity.

Please see the bug report for more details.

Change-Id: Ifc76f2b1cce834b3c9927359ac9b957bc9f9c65f
Closes-Bug: #1326183
Co-Authored-By: Dan Smith <dansmith@redhat.com>
Merge "Neutron: Atomic update of instance info cache"