Add option to allow cross AZ attach configurable

Make check_attach() optionally check if the volume and instance
are in the same availability zone and if cross AZ attach
is configured as not allowed report error.

This does not change the current default behaviour.

DocImpact: Adds a new Nova Config option

Change-Id: Ib0e085888b1c6620869261d87cd964de302accb3
Merge "Add option to allow cross AZ attach configurable"