Fix `repo.saltstack.com` URLs for pinned minor release (#364)

* All minor releases appear in an `archive` sub-directory.
* Resolves main bug in GitHub issue #364.
* Simplification of original GitHub PR #365 by @ralucasg.

salt/map.jinja

@@ -41,6 +41,12 @@ that differ from whats in defaults.yaml
 ##}
 {% set osrelease = salt['grains.get']('osrelease') %}
 {% set salt_release = salt['pillar.get']('salt:release', 'latest') %}
+
+{# All minor releases appear in an `archive` sub-directory #}
+{% if salt_release.split('.')|length >= 3 %}
+{% set salt_release = 'archive/' ~ salt_release %}
+{% endif %}
+
 {% set os_family_map = salt['grains.filter_by']({
     'Debian':  {
       'pkgrepo': 'deb http://repo.saltstack.com/apt/' +