barectf: schemas: use conditionals intead of `oneOf` when possible

barectf: schemas: use conditionals intead of `oneOf` when possible

Using `if`/`then`/`else` instead of `oneOf` makes the `jsonschema`
validator create much more readable errors.

For example:

Before:
    Error:
      Configuration: Cannot create configuration from YAML file
        `config.yaml`
      Configuration object:
      `metadata` property:
      `clocks` property:
      `some_clock` property:
      `offset` property:
      `seconds` property: -2 is not valid under any of the given
        schemas: -2 is less than the minimum of 0; -2 is not of type
        'null' (from schema `2/config/config`)

Now:
    Error:
      Configuration: Cannot create configuration from YAML file
        `config.yaml`
      Configuration object:
      `metadata` property:
      `clocks` property:
      `some_clock` property:
      `offset` property:
      `seconds` property: -2 is less than the minimum of 0
        (from schema `2/config/config`)

This is because, with conditionals, we assume that the user intended
something with some valid schema, and add more constraints. The example
above is for an integer having a minimum value of 0, or a null value. As
soon as we know that the value is an integer, then we know its minimum
must be 0; we decide to not care about the fact that the whole value
could also be null (which the old message indicated). In my opinion,
this is a better UX.

The example above is simple. For more complex schemas, the UX gain is
even more obvious.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>