Config rehaul

[dmp42]

This is mainly to fix our various problems with configuration typage/env that grew sore in the aftermath of the standalone True debacle:

• None typage for unset config object lead to unexpected behavior #423
• config.standalone is not loaded as True, but as 'True' causing issues #431 (finally making @petarmaric happy)

Incidentally, this also lay code to fix #440 and does fix #442.

BEWARE that the Configuration object behavior changed and that this may have unexpected and tricky impact, so, please review (ping my favorites-eagle-eyed reviewers @shin- and @wking) and if possible test.

Please note I'm not changing for the sake of it - I'm changing so that we have something that behaves rationally - and that we know exactly how.

In a nutshell:

• environment derived values are now typed properly (eg: they are YAML parsed) - which means MY_ENV_VAR=True will give True and MY_ENV_VAR='["foo", "bar"]' will give a python list, etc
• default values (for env derived values) are now typed properly (same as above)
• the config object is now usable with dot notation, as deep as there is values. eg: cfg.foo.bar.baz
• the config object is no longer usable with the get method
• dict values are themselves instances of the Config class
• a non existent key with return None
• falsy values are no longer coalesced
• the common flavor is no longer forcefully inherited inside python code, but the heritage is declared using YAML - this is DEFINITELY a point where you need to update configuration files and that will break for people not paying attention
• as long as a key exists in YAML, it WILL be returned (eventually None) - this is an important change from the previous behavior where falsy values would mean the key is not returned at all (propagating to the parent dict which in turn might have returned None previously if no key were Trusy). This is especially true for the mirroring and cache config (code has been adjusted)
• we now have extensive tests to characterize all this
• default values were previously scattered (half in code, half in config.yaml). This is no longer the case, and all defaults are now specified in the config file. This WILL break for people not paying attention, specially the redis code which no longer makes "default" assumptions

@shin-: do we need config/config_mirroring.yml anymore? or can I remove it?

[petarmaric]

Thanks for keeping me happy 👍

Nice work, this pull request LGTM.

[mdub]

Nice! 👍

[dmp42]

@shin- ping

[shin-]

LGTM

[dmp42]

Thanks @shin- !
@wking would you have time to exert your x-rays on this before it gets in and mistakenly hurt unicorns?