Remove non-Salted external apt sources

Workaround saltstack/salt#26605
by cleaning the sources.list.d folder first, and running
pkgrepo states that add repositories to the folder afterwards
using require.

This unbreaks #232: Travis has the Chrome apt repo configured, but
Google has dropped 32-bit support (March 2016), causing apt to fail
on multiarch hosts. By removing all external repos we will only use
repositories we configure ourselves.

ubuntu/init.sls

@@ -6,3 +6,18 @@
     - group: root
     - mode: 755
     - source: salt://{{ tpldir }}/files/policy-rc.d
+
+# Workaround for https://github.com/saltstack/salt/issues/26605
+# Clean the directory first, and require it in all pkgrepo states
+# which add repositories to the sources.list.d folder (instead of
+# the main /etc/apt/sources.list file)
+/etc/apt/sources.list.d:
+  file.directory:
+    - user: root
+    - group: root
+    - dir_mode: 755
+    - recurse:
+      - user
+      - group
+      - mode
+    - clean: True