Adds a configuration option [iptables]ip_version to specify the
desired ip version for the iptables pxe filter, which can be set to
4 or 6. When set to 6, the iptables pxe filter will use ip6tables
command to manage rules for the port 547 which is the port of DHCPv6
The string type is used to make room for the future, when there is need
to automatically determine ip version from the binding interface.
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).