slapos/recipe/slapconfiguration: Propagate SR defaults to configuration dict

Currently in slapos / slapconfiguration-recipe we have 2 ways to access instance parameters:

configuration dict,

and

configuration.<key>

for every valid key in configuration dict.

An SR could also provide default value for a key, e.g. like helloworld SR does for name parameter:

https://lab.nexedi.com/nexedi/slapos/blob/3c552c05/software/helloworld/instance.cfg.in#L38

and if such parameter is not provided in instance parameters, the default will be used...

... Though currently it only works for configuration. way of access: if a default value was provided by SR with

configuration.key = ... 

it will be available via configuration.key, but configuration dict will not have key entry at all.

This looks non-consistent to me, so imho in addition to propagating

configuration {} -> configuration.<key>

on parameters receive, we should also propagate

configuration.<key> -> configuration {} defaults

so that at any time two pictures (configuration {} and set of configuration.) agree between each other.

NOTE The fix also works for slapconfiguration.serialised case, where we obtain parameters as { _ -> json } and json-decode them to python dict after receive.

The reason it works is that we apply defaults after parameters
decode, so e.g. the following

# in SR
configuration.name = John Doe 

# instance parameters
_ = { "aaa": "bbb" }

will result in the following `configuration` dict:

{ 'aaa': 'bbb', 'name': 'John Doe' }

/cc @vpelletier, @alain.takoudjou, @rafael, @Tyagov