Units handling in the Catalog

We had several options how to implement that:

* one plan and user provided json to specify number of units: we don't
like this option
* multiple plans and we hardcode number of units based on plan name
which means we have to change broker code every time the plan changes :
nah...

* fork brokerAPI and modify the Catalog struct and do PR upstream: it's
hard to extend this in a generic way by using json.RawMessage. The name
of the field has to be known and specified as json unmarshaller option.
So we would have to specify `json:"units,omitempty"` json unmarshaler
option which is very compose specific.
There is a go lang issue opened to fix that:
golang/go#15314

* do not use upstream Catalog and implement our own struct. Not possible
- we have to return the type specified in the brokerapi interface to
implement it:
https://github.com/pivotal-cf/brokerapi/blob/0d62e62ec041b8ff39c0e304f437b4eabb4175fa/service_broker.go#L10

* extend external Catalog struct to include units field - not possible.
Same reason as above

I had to implement the sixth option. Now we have a new struct that
partially copies upstream catalog structure but holds only fields (IDs
and Units ) that we care about. This together with upstream catalog is
wrrapped into one struct and stored in main broker structure so all
interface methods have access to it. There is also new catalog.Load method
that replaces catalog.New. It has two unmarshalling passes to fill
upstream catalog and our Units struct.

catalog/catalog.go

@@ -2,20 +2,80 @@ package catalog
 
 import (
 	"encoding/json"
+	"fmt"
 	"io"
+	"io/ioutil"
 
 	"github.com/pivotal-cf/brokerapi"
 )
 
+type ComposeCatalog struct {
+	Catalog      Catalog
+	ComposeUnits ComposeUnits
+}
+
+// Catalog is an upstream Catalog struct
 type Catalog struct {
 	Services []brokerapi.Service `json:"services"`
 }
 
-func New(catalog io.Reader) (*Catalog, error) {
-	cp := &Catalog{}
-	err := json.NewDecoder(catalog).Decode(cp)
+type ComposeUnits struct {
+	Services []Service `json:"services"`
+}
+
+type Service struct {
+	ID    string        `json:"id"`
+	Name  string        `json:"name"`
+	Plans []ServicePlan `json:"plans"`
+}
+
+type ServicePlan struct {
+	ID       string              `json:"id"`
+	Metadata ServicePlanMetadata `json:"metadata,omitempty"`
+}
+
+type ServicePlanMetadata struct {
+	Units int `json:"units,omitempty"`
+}
+
+func (c *ComposeCatalog) GetService(id string) (*Service, error) {
+	for _, service := range c.ComposeUnits.Services {
+		if service.ID == id {
+			return &service, nil
+		}
+	}
+
+	return nil, fmt.Errorf("service: not found")
+}
+
+func (s *Service) GetPlan(id string) (*ServicePlan, error) {
+	for _, plan := range s.Plans {
+		if plan.ID == id {
+			return &plan, nil
+		}
+	}
+
+	return nil, fmt.Errorf("plan: not found")
+}
+
+func (c *ComposeCatalog) Load(catalog io.Reader) error {
+	buf, err := ioutil.ReadAll(catalog)
 	if err != nil {
-		return nil, err
+		return err
 	}
-	return cp, nil
+
+	err = json.Unmarshal(buf, &c.Catalog)
+	if err != nil {
+		return err
+	}
+
+	err = json.Unmarshal(buf, &c.ComposeUnits)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+func (c *ComposeCatalog) Validate() error {
+	return nil
 }