recursive connection_options with merged extra support

[viktorkertesz]

Dear Nornir community,
I'm curious about your opinion on the following matter:

Problem

Currently connection_options.extras is a dict and the logic to access this attribute is that Host object extras is the top priority and then Group and Defaults follows. The extras attribute is simply overwritten in more specific configuration. However it would be nice to have this attribute as a merge of all the other extras defined in Host, Group and Defaults instances.

Possible solution logic

*First, I'd like to highlight that this behavior changes the API and must be communicated and use in upcoming main version!*

• Rewrite of get_connection_options logic in a way that defaults, group and host connection_options.extra parameters would be merged. Priority is defaults -> group1, group2,... -> host
  So a host defined extra parameter would override defaults or groups if the key is the same. All distinct parameters will be merged together.
• Rewrite connection_options in Host class as property and alter access of this attribute to support recursion in runtime.
• Figure out how can we 'remove' certain parameter if we want to delete something instead of merge. Maybe a None value would do the trick.

Benefits

We can better segment our connection_options extra parameters. We can define partial extras in defaults, groups and hosts.

Downside of the new logic

If a user relies on that host defined extra will not merge but replace the default/groups, then the user might get additional parameters which might be unexpected.

Example

defaults.yaml:

---
connection_options:
    netmiko:
        extras:
              secret: nornir

groups.yaml:

distant:
    connection_options:
        netmiko:
            extras:
                  timeout: 4

hosts.yaml:

host1:
    groups:
         - distant
    connection_options:
        netmiko:
            extras:
                  port: 222

Old behavior

This will result in that host1 will NOT have the secret parameter nor the timeout value. Host file replace the extras.

New behavior

host1 will have set the port 222, timeout of 4s and the secret set as well. Host file will merge the extras.

TODO

• I added connection_options extra gathering in a merged way #759 but it is still very early and not complete.
• more comprehensive tests with more realistic inventory
• Complete docs on the behavior. (Currently there is no docs at all on this part of the inventory, not even for the original behavior)