Add support for defining labels in Helm Broker bundles

[mszostok]

Description

Changes proposed in this pull request:

• Add support for defining labels in Helm Broker bundles
• Fix behaviour of schema population in catalog.go: if schema not provided in bundle then the empty schema is not exported to Service Catalog but the object is still nil.

Related issue(s)

• Helm Broker bundles marked as local and/or showcase #1239

[mszostok]

FYI

[mszostok]

FYI: hardcoded here, can be replaced with

for k, v := range overridesForPlanLabels {
	metaCopy.Labels[k]  = v
}

where globaly we have

var overridesForPlanLabels = map[string]string{
        // Business requirement that helm bundles are always treated as local
	"local": "true",
}

but IMO for now it's not necessary.

[mszostok]

Refactor inspired by golden testing, see: https://golang.org/src/go/doc/doc_test.go#L119

[mszostok]

I tried to assert on Go obj
using sth like that:

func fixtureOsbService(t *testing.T, testdataBasePath string) osb.Service {
	data, err := ioutil.ReadFile(testdataBasePath)
	require.NoError(t, err, "failed reading .golden")

	out := osb.Service{}
	require.NoError(t, json.Unmarshal(data, &out))
	return out
}

Unfortunately kinds are different, e.g. map[string]string vs Labels
where Labels is type Labels map[string]string. Then in the assertion, it cannot be compared easily. Because of that, I decided to normalize it to the JSON form.

[piotrmiskiewicz]

lgtm