Deprecation warning in resolve_config suggests to write None instead of none into config file

[vmichals]

Describe the bug
The deprecation warning raised here when using a config with the following content:

algorithms:
    random:
        seed: null

suggests to change the config to the following:

experiment:
  algorithms: {'random': {'seed': None}}

The None should be null in yaml/json files.

Expected behavior
The deprecation warning should show null (proper yaml syntax).

Steps to reproduce
See the example config in the bug description above

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Python version: 3.6.9
• Oríon version: 0.1.7
• Database: PickleDB

Possible solution
See PR #391

Instead of calling repr(backward_value) use yaml.dump(backward_value) here

[Thomsch]

Thank you for reporting this bug!
We'll take a look at your solution as soon as possible!

Thanks again!