syscontainers: drop sha256 only if prefix

After this fix, we can install a system container by the image sha256 ID:

atomic install --system busybox@sha256:817a12c32a39bbe394944ba49de563e085f1d3c5266eb8e9723256bc4448680e

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #890
Approved by: baude

Atomic/syscontainers.py

@@ -1287,7 +1287,9 @@ def _get_ostree_image_branch(img):
         if "ostree:" in img:
             imagebranch = img.replace("ostree:", "")
         else: # assume "oci:" image
-            imagebranch = "%s%s" % (OSTREE_OCIIMAGE_PREFIX, SystemContainers._encode_to_ostree_ref(img.replace("sha256:", "")))
+            if img.startswith("sha256:"):
+                img = img.replace("sha256:", "", 1)
+            imagebranch = "%s%s" % (OSTREE_OCIIMAGE_PREFIX, SystemContainers._encode_to_ostree_ref(img))
         return imagebranch
 
     def has_image(self, img):