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.
Yep, we want to check the exact number of calls, at least for the moment. See comment below about the WIP pull request: there is a test that validates that images have properly been registered in the image cache and we need to make sure the strategy is not being called again.