Install python-treq from strech on jessie nodes

New soledad-common depends on `python-treq`, which
is only available in debian stretch.
We pin all stretch packages to 1 (same as for sid), which
means (from `man apt_preferences`):

    "causes a version to be installed only if there is no
    installed version of the package"

- Resolves: #8836

puppet/modules/site_apt/manifests/init.pp

@@ -20,11 +20,14 @@ class site_apt {
     $apt_platform_codename = $platform_sources['apt']['codename']
   }
 
-  # needed on jessie hosts for getting pnp4nagios from testing
+  # needed on jessie hosts for getting python-treq from stretch
+  # see https://0xacab.org/leap/platform/issues/8836
   if ( $::operatingsystemmajrelease == '8' ) {
     $use_next_release   = true
+    $custom_preferences = template("site_apt/${::operatingsystem}/preferences_jessie.erb")
   } else {
     $use_next_release   = false
+    $custom_preferences = ''
   }
 
   class { 'apt':
@@ -33,6 +36,7 @@ class site_apt {
     security_url       => $apt_url_security,
     backports_url      => $apt_url_backports,
     use_next_release   => $use_next_release,
+    custom_preferences => $custom_preferences,
     repos              => 'main'
   }
 

puppet/modules/site_apt/templates/Debian/preferences_jessie.erb

+Explanation: Debian jessie
+Package: *
+Pin: release o=Debian,n=jessie
+Pin-Priority: 990
+
+Explanation: Debian stretch
+Package: *
+Pin: release o=Debian,n=stretch
+Pin-Priority: 1
+
+Explanation: Debian sid
+Package: *
+Pin: release o=Debian,n=sid
+Pin-Priority: 1
+
+Explanation: Debian fallback
+Package: *
+Pin: release o=Debian
+Pin-Priority: -10