Use value from ContextDetails to populate Namespace (#373)

* Use value from ContextDetails to populate Namespace

This is a fix for: #372

This change uses the value from ContextDetails.Namespace to populate
KubernetesClientConfiguration.Namespace.

The issue is there's a Namespace property on both Context and ContextDetails
 - The property on Context is used today
 - The property on ContextDetails is not
 - The property on ContextDetails maps to the actual yaml config

* Obsolete Context.Namespace

This property doesn't map to anything in the YAML and thus will never be
set. Other clients I checked (java, golang) don't look for a property
at this level.

I think this was likely a mistake, and it should be obsoleted because
it will never be populated.

Example:

```yaml
contexts:
- context:
    cluster: ...
    namespace: ... # this is ContextDetails.Namespace
    user: ...
  name: foo
```

```yaml
contexts:
- context:
    cluster: ...
    namespace: ...
    user: ...
  name: foo
  namespace: ... # this is Context.Namespace
```

src/KubernetesClient/KubeConfigModels/Context.cs

@@ -1,5 +1,6 @@
 namespace k8s.KubeConfigModels
 {
+    using System;
     using YamlDotNet.Serialization;
 
     /// <summary>
@@ -19,6 +20,7 @@ namespace k8s.KubeConfigModels
         [YamlMember(Alias = "name")]
         public string Name { get; set; }
 
+        [Obsolete("This property is not set by the YAML config. Use ContextDetails.Namespace instead.")]
         [YamlMember(Alias = "namespace")]
         public string Namespace { get; set; }
     }