handle empty network info in instance cache

The nw_info value [] was being incorrectly equated with None. This caused
infinite recursion when api._get_instance_nw_info returned [] and was wrapped
with @refresh_cache (e.g., _get_instance_nw_info in
nova.network.neutronv2.api.API).

Fixes bug #1251792.

Change-Id: I3034fbb20e790f2d6f22c50b74a9f9dcdf9081aa
xenapi: move session into new client module

This is the start of building a set of objects on top of the XenAPI
library to make a clear separation between XenAPI client and
business logic.

This is all the first steps towards pull out the client lib into oslo
incubator, so we can share code between Nova and Cinder.

DocImpact
While there is a change to the sample config file, there is no change in
the content of the config file.

Part of blueprint xenapi-driver-refactor
Change-Id: I463923a9ee289ff4b86fcf26d8c656609685857e
Sync Log Levels from OSLO

Copy changes from just 1 commit to reduce size of n-cpu
logs reported by infra folks

cbfded9c Default iso8601 logging to WARN

Change-Id: I71a51374c58001380c52d9c577d257f98b880d87
Merge "xenapi: move session into new client module"
Merge "Sync Log Levels from OSLO"
Merge "handle empty network info in instance cache"