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.<key> 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.<key>) 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

slapos/recipe/slapconfiguration.py

@@ -108,6 +108,12 @@ class Recipe(object):
   OPTCRE_match = RawConfigParser.OPTCRE.match
 
   def __init__(self, buildout, name, options):
+      # propagate configuration.<key>  ->  default configuration {}
+      parameter_default_dict = {}
+      for key, value in options.iteritems():
+        if key.startswith('configuration.'):
+          parameter_default_dict[key[len('configuration.'):]] = value
+
       parameter_dict = self.fetch_parameter_dict(options,
                                       buildout['buildout']['directory'])
 
@@ -116,6 +122,12 @@ class Recipe(object):
       # If it is non-dict - we just store it as is;
       # if it is     dict - we process it further.
       if isinstance(parameter_dict, dict):
+        # apply default parameters to parameter_dict
+        for key, value in parameter_default_dict.iteritems():
+          if key not in parameter_dict:
+            parameter_dict[key] = value
+
+        # propagate configuration {} -> configuration.<key>
         match = self.OPTCRE_match
         for key, value in parameter_dict.iteritems():
             if match(key) is not None: