Some commands related to multiple backends will have unnecessary calls from one backend

[peterbaouoft]

There are three cases that I have found so far which involves unnecessary calling of functions:

1: when listing images with filter: currently, our method for handling atomic images list -f is that we first have a list of all the images(no matter the backend), then we choose some of the images that matches the filter criteria to display. In this case, some of the function calls will be unnecessary if the filter was to filter the backend type. For instance, atomic images list -f type=ostree should not call docker related functions(may be only need to get all the images from ostree backend)

2: When there are no images in one specific backend: When no images existing in both backend, the functions for both backend involving with images are still being called.

3: Similarly, when there is no containers in one specific backend: In that case, functions from both backend for finding containers will also get called