Parse backend options in backend (#3227)

Currently, backend options are parsed in the yaml parser.
This has some issues:
- backend specific code should be in the backend folders
- it is not possible to add backend options for backends added via
addons

pipeline/frontend/yaml/compiler/compiler.go

@@ -34,7 +34,6 @@ type Registry struct {
 	Hostname string
 	Username string
 	Password string
-	Email    string
 	Token    string
 }