snap-tool: prefer 'stable' channel over 'stable/ubuntu-X.X' for core snaps

When the "snap-tool" script encounters a core snap it will switch to
the "stable" channel if the channel was set to "stable/ubuntu-X.X". This
ensures that core snaps always come from one canonical source.

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.573) UNRELEASED; urgency=medium
+
+  * When the "snap-tool" script encounters a core snap it will switch to
+    the "stable" channel if the channel was set to "stable/ubuntu-X.X". This
+    ensures that core snaps always come from one canonical source.
+
+ -- Tobias Koch <tobias.koch@canonical.com>  Wed, 20 Mar 2019 14:33:34 +0100
+
 livecd-rootfs (2.572) disco; urgency=medium
 
   * Revert "subiquity: fix name of mdoules.squashfs to mount", debian-cd

snap-tool

@@ -24,6 +24,7 @@ import getopt
 import hashlib
 import json
 import os
+import re
 import shutil
 import subprocess
 import sys
@@ -150,6 +151,14 @@ class Snap:
         if self._details and not cohort_create:
             return self._details
 
+        if re.match(r"^core\d*$", self._name) and \
+                self._channel.startswith("stable/ubuntu-"):
+            sys.stderr.write(
+                "WARNING: switching channel from '{}' to 'stable' for '{}' "
+                "snap.\n".format(self._channel, self._name)
+            )
+            self._channel = "stable"
+
         path = "/v2/snaps/refresh"
 
         data = {