Use real types to parse labels

We were using `yaml.Unmarshal` on basic types or inline structs. This
was not efficient and not clear to defined what we expect in labels.
We now use types to unmarshal the labels.

Only the `values` label is, at this time, parsed by GetValuesFromLabel
because this `utils` function is clearly a special case.

generator/ingress_test.go

@@ -19,7 +19,7 @@ services:
         - 443:443
         labels:
             %s/ingress: |-
-                host: my.test.tld
+                hostname: my.test.tld
                 port: 80
 `
 	composeFile = fmt.Sprintf(composeFile, katenaryLabelPrefix)