JCLOUDS-570: Fallback to the GetImageStrategy in TemplateBuilderImpl

If the TemplateBuilderImpl is given an imageId but the image can not be
found in the image cache, fallback to the GetImageStrategy to perform a
call to the provider to try to get it.

We've seen that in some cases images are not returned in the image list
but they actually exist in the provider. This fix won't make them
available when filtering by other properties such as the operating system,
etc, but at least will make them available if their id is known.
[minor] We can save a bit of object construction at the cost of a little more complexity:
```
ImageThing imageFromProvider = getImageStrategy.getImage(imageId);
if (imageFromProvider == null) {
   ...
}
image = Optional.of(imageFromProvider); // or just return imageFromProvider
```
What do you think?