dockerng: fix image_present for forced, non-existent image
dockerng: fix image_present for forced, non-existent image

With a non-existent image, which is forced, the call to `inspect_image`
will raise a `CommandExecutionError`: "Unable to get info for image".

This commit changes it to only call `inspect_image` for existing images.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt