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.
Are we interested in how exactly this call is made? We know _some_ call is made because the result is correct...but would we care if e.g. _two_ calls were made?