discovery service: dynamically replace 0.0.0.0 endpointUrl in server response

[brubbel]

cfr. #680

If the opcua server is configured with the default endpoint 'opc.tcp://0.0.0.0:4840',
the discovery service will dynamically replace the endpointUrl with the url provided in the FindServersParameters request of the client. e.g. 'opc.tcp://192.168.10.123:4840'
If the opcua server is configured with a valid endpoint url, configuration remains untouched.

Solves the following issues:

1. Fixes the "Replaced Hostname" warning in UaExpert, even when the opcua server is listening on multiple interfaces.
   
2. Makes freeopcua compatible with OPC Scout (and possibly also with WinCC) if a certificate is used.
3. Provides a simple solution for a valid endpointUrl for NAT'ed systems (vpn etc), without a global discovery server.

[brubbel]

Will need second pair of eyes for this :-)
import ipaddress only works for Py3. Is it acceptable to add a dependency to py2-ipaddress (pip install ipaddress) for Py2?

[oroulet]

what is that? why dumping then loading again?

[brubbel]

shallow copy() does not copy the endpoints, so either deepcopy() or pickle...

[oroulet]

The first version was simple, but the last commit really complicate things... I am afraid to add so much code jsut to replace 0.0.0.0 for clients when this can simply be done by configuring the server

[brubbel]

Ok, but the problem is still there when listening on multiple interfaces, or when the server is using dhcp, or the client is using VPN. Maybe it's better to let the PR pending for a while until it's all worked out nicely?
(Things became complicated when involving IPv6).

Goal is ultimately to support opc.tcp://nicehostname:4840/freeopcua/server without any additional IP stuff.

[oroulet]

yes using deepcopy is clearly the correct way to implement this ;-)