Find a better way to specify default value for CI config entry node

The issue was raised from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9666#note_24549599

The background is that we want some default values for .gitlab-ci.yml, for example the following config:

job0:
  script: pwd

Should be interpreted as if: (just a simplified example, not how it's working right now)

job0:
  script: pwd
  stage: test

That is if stage is not specified, it would use a default value. To retrieve the value, we do something like this:

job = Gitlab::Ci::Config::Entry::Factory.new(Gitlab::Ci::Config::Entry::Job)
  .value({script: 'pwd'}).metadata(name: :job0).with(key: :job0).create!
job.compose!
job.value # => {:script=>["pwd"], :name=>:job0, :commands=>"pwd", :stage=>"test"}

How this was currently implemented was actually in Job#value:

def value
  @config.merge(to_hash.compact)
end

# Simplified, not actual code
def to_hash
  { name: name,
    script: script_value,
    commands: commands,
    stage: stage_value }
end

If we remove that stage in to_hash, then the default stage test won't be set. The reason resides in Node#value:

def value
  if leaf?
    @config
  else
    meaningful = @entries.select do |_key, value|
      value.specified? && value.relevant?
    end

    Hash[meaningful.map { |key, entry| [key, entry.value] }]
  end
end

Here the value would only pick specified? and relevant? nodes. However in the original example, stage is not specified, which is a Unspecified node wrapping around Stage node. It's not specified therefore it's not populated in the value. It's only populated at the moment because Node#value is overridden by Job#value.

---

For now, I am doing the same for Cache:

def value
  super.merge(key: key_value)
end

Ideally, we should probably not override any value method, and have a way to populate default values.

Note that Node.default is only used to populate the node whenever it's an Unspecified node, but the particular node is not populated in Node#value. This is a bit confusing. If it's never picked in Node#value, why are we setting the default value? It's because Job#value is using it. Now also Cache#value would be using it.

We should also be careful that we should not break gitlab-ce#18775

---

It would be nice if we have an API like this:

entry :stage, Entry::Stage, unspecified: :default,
  description: 'Pipeline stage this job will be executed into.'

Or maybe just make this default, so we don't tell the difference if it's unspecified or having a default. Might need to explore a bit.

/cc @grzesiek @ayufan