UPSTREAM: 31353: fix duplicate validation/field/errors

Related PR: openshift/origin#10317
Related BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1257800#c3

PR kubernetes#10317 fixed duplicate errors for invalid aggregate errors in
https://github.com/openshift/origin/blob/master/vendor/k8s.io/kubernetes/pkg/kubectl/cmd/util/helpers.go.

However, duplicate aggregate errors that went through
https://github.com/openshift/origin/blob/master/vendor/k8s.io/kubernetes/pkg/util/validation/field/errors.go
were not affected by that patch.

This patch adds duplicate aggregate error checking to
`pkg/util/validation/field/errors.go`

\##### Before
`$ oc set env rc/idling-echo-1 test-abc=1234`
```
error: ReplicationController "idling-echo-1" is invalid:
[spec.template.spec.containers[0].env[0].name: Invalid value:
"test-abc": must be a C identifier (matching regex
[A-Za-z_][A-Za-z0-9_]*): e.g. "my_name" or "MyName",
spec.template.spec.containers[1].env[0].name: Invalid value: "test-abc":
must be a C identifier (matching regex [A-Za-z_][A-Za-z0-9_]*): e.g.
"my_name" or "MyName", spec.template.spec.containers[0].env[0].name:
Invalid value: "test-abc": must be a C identifier (matching regex
[A-Za-z_][A-Za-z0-9_]*): e.g. "my_name" or "MyName",
spec.template.spec.containers[1].env[0].name: Invalid value: "test-abc":
must be a C identifier (matching regex [A-Za-z_][A-Za-z0-9_]*): e.g.
"my_name" or "MyName"]
```

`$ oc set env rc/node-1 test-abc=1234`
```
error: ReplicationController "idling-echo-1" is invalid:
[spec.template.spec.containers[0].env[0].name: Invalid value:
"test-abc": must be a C identifier (matching regex
[A-Za-z_][A-Za-z0-9_]*): e.g. "my_name" or "MyName",
spec.template.spec.containers[1].env[0].name: Invalid value: "test-abc":
must be a C identifier (matching regex [A-Za-z_][A-Za-z0-9_]*): e.g.
"my_name" or "MyName"]
```

\##### After
`$ oc set env rc/idling-echo-1 test-abc=1234`
```
error: ReplicationController "idling-echo-1" is invalid:
[spec.template.spec.containers[0].env[0].name: Invalid value:
"test-abc": must be a C identifier (matching regex
[A-Za-z_][A-Za-z0-9_]*): e.g. "my_name" or "MyName",
spec.template.spec.containers[1].env[0].name: Invalid value: "test-abc":
must be a C identifier (matching regex [A-Za-z_][A-Za-z0-9_]*): e.g.
"my_name" or "MyName"]
```

`$ oc set env rc/node-1 test-abc=1234`
```
error: ReplicationController "node-1" is invalid:
spec.template.spec.containers[0].env[0].name: Invalid value: "test-abc":
must be a C identifier (matching regex [A-Za-z_][A-Za-z0-9_]*): e.g.
"my_name" or "MyName"
```

:100644 100644 203f7cc... 089f21c... M	pkg/util/validation/field/errors.go

Author: juanvallejo

pkg/util/validation/field/errors.go

@@ -22,6 +22,7 @@ import (
 	"strings"
 
 	utilerrors "k8s.io/kubernetes/pkg/util/errors"
+	"k8s.io/kubernetes/pkg/util/sets"
 )
 
 // Error is an implementation of the 'error' interface, which represents a
@@ -201,9 +202,15 @@ func NewErrorTypeMatcher(t ErrorType) utilerrors.Matcher {
 
 // ToAggregate converts the ErrorList into an errors.Aggregate.
 func (list ErrorList) ToAggregate() utilerrors.Aggregate {
-	errs := make([]error, len(list))
-	for i := range list {
-		errs[i] = list[i]
+	errs := make([]error, 0, len(list))
+	errorMsgs := sets.NewString()
+	for _, err := range list {
+		msg := fmt.Sprintf("%v", err)
+		if errorMsgs.Has(msg) {
+			continue
+		}
+		errorMsgs.Insert(msg)
+		errs = append(errs, err)
 	}
 	return utilerrors.NewAggregate(errs)
 }