Add a three-way-handshake on autoregistration procedure or improve the error messages on invalid DATASERVICE_HOST

[alexbarcelo]

[Note: Maybe this also affects Java, I have only realized this problem with Python Execution Environment]

The current behaviour when the Execution Environment is loading is that it performs an "autoregister" procedure with the Logic Module. This is successful as long as the LogicModule host is correct.

However, the LogicModule needs to be able to connect to the DataService, and sometimes this may be non-straightforward (e.g. in hybrid docker deployments, in the presence of certain NAT tables or firewalls or with other network shenanigans). The solution (which already is working!) is to use the DATASERVICE_HOST configuration field.

Everything works, if properly configured. The usability problem is the following: if DATASERVICE_HOST is improperly set (because you forgot or because you messed up its value) then the initialization seems to work but the system will break on the model registration with a somewhat cryptic java.lang.RuntimeException: UNAVAILABLE: io exception. We may want to know as soon as possible (i.e. during autoregistration) that the DataService is improperly configured and yield an error at that point. That can be done with a three-way-handshake strategy. If that proves challenging, we can instead improve the error, or add some validation background thing or something.

This only affects usability, so it is low priority, but it's worth keeping that in mind in case we are modifying some registration aspects in the future.