These calls are subject to transient network problems, we should
not abort ironic-inspector process in this case. Also due to
bug 1748893 the port listing API can sometimes return HTTP 400.
This change retries port listing 5 times with 1 second break
before aborting the periodic task and thus the process.
This change introduces a dependency on the retrying library,
which is already widely used in OpenStack (including ironic).
Change-Id: I92fd70ca5692ce9f6798eedf9e540d5aa7c6f1af
Closes-Bug: #1748893
(cherry picked from commit 3237511cc6)