Allow to override the Image and Security extension bindings

Some providers, such as Azure ARM, might need additional
resources present in order to be able to use the extension.
This change allows to override its binding (Guice 4 by default does not
allow to override provider bindings, so we need to define them as final
ones and provide an alternate method for subclasses that need to
override the binding).

compute/src/main/java/org/jclouds/compute/config/BaseComputeServiceContextModule.java

@@ -304,16 +304,22 @@ public abstract class BaseComputeServiceContextModule extends AbstractModule {
    @Provides
    @Singleton
    public final Optional<ImageExtension> guiceProvideImageExtension(Injector i) {
-      Binding<ImageExtension> binding = i.getExistingBinding(Key.get(ImageExtension.class));
-      return binding == null ? Optional.<ImageExtension> absent() : Optional.of(binding.getProvider().get());
+      return provideImageExtension(i);
    }
 
    @Provides
    @Singleton
    protected final Optional<SecurityGroupExtension> guiceProvideSecurityGroupExtension(Injector i) {
+      return provideSecurityGroupExtension(i);
+   }
+
+   protected Optional<ImageExtension> provideImageExtension(Injector i) {
+      Binding<ImageExtension> binding = i.getExistingBinding(Key.get(ImageExtension.class));
+      return binding == null ? Optional.<ImageExtension> absent() : Optional.of(binding.getProvider().get());
+   }
+
+   protected Optional<SecurityGroupExtension> provideSecurityGroupExtension(Injector i) {
       Binding<SecurityGroupExtension> binding = i.getExistingBinding(Key.get(SecurityGroupExtension.class));
       return binding == null ? Optional.<SecurityGroupExtension> absent() : Optional.of(binding.getProvider().get());
    }
-   
-
 }