Update the testing matrix to cover most distro+salt+python combinations

Use py3 for latest salt version
Use Python 2 on old distros
Update Gemfile
Updated centos-6 run_command, as it does not use systemd
Added tests to verify correct package versions for all cases tested

salt/pkgrepo/suse/absent.sls

@@ -0,0 +1,3 @@
+drop-saltstack-pkgrepo:
+  pkgrepo.absent:
+    - name: saltstack-pkgrepo

salt/pkgrepo/suse/init.sls

@@ -0,0 +1,10 @@
+{% from "salt/map.jinja" import salt_settings with context %}
+
+saltstack-pkgrepo:
+  pkgrepo.managed:
+    - name: systemsmanagement_saltstack_products
+    - humanname: SaltStack repo for Opensuse 42.3
+    - baseurl: {{ salt_settings.pkgrepo }}
+    - enabled: 1
+    - gpgcheck: 1
+    - gpgkey: {{ salt_settings.key_url }}