Address infinite loop in nova compute when getting network info

Move the refresh_cache decorator to get_instance_nw_info.
In fact, _get_instance_nw_info is called by the decorator
itself, potentially causing an infinite loop in case nw_info
is not of the expected value. This is also inline with
the method's purpose, as stated by the docstring. At the
same time, ensure that delete_port_for_instance and
allocate_for_instance continue to refresh the cache by
using the decorated version.

Also, add a couple of debug traces that are friendly to
the developer.

Partial-bug: 1235435
Related-bug: 1251792

Change-Id: I06f8634ea241d05ac8fbcc290adf0cb23829f3e4
(cherry picked from commit 2e520496c369a8ef646820ec068cc4416ab50eca)