[podman] Simplify the code for checking if an image exists

Let's switch to the existing 'podman image exists' to determine if
an image is local to the machine or if it needs pulling.

Let's change this for a number of reasons:

• It simplifies the code greatly
• It makes everything work even when the image is prefixed with 'localhost/' names which are not detected as local images
• It makes the code faster as we do not need to list all the images as that is more expensive

Tested by running a couple of full containerized OpenStack deployments,
one with images tagged as 'localhost/foo/mysql:pcmklatest' and one
with a normal registry host prefix.

Co-Authored-By: Damien Ciabrini <dciabrin@redhat.com>