test(inspec): enable use_upstream_repo for debian & centos-6

* Add new `centos-6` image
* Install `9.6` from upstream repo

kitchen.yml

@@ -53,15 +53,8 @@ platforms:
       provision_command:
         - zypper refresh && zypper install -y net-tools
       run_command: /usr/lib/systemd/systemd
-  # centos-6 guest fails on Debian hosts due to vsyscall issues, see
-  # https://hub.docker.com/_/centos, "A note about vsyscall"
-  # Disabled for `template-formula` because not `systemd` based
-  # - name: centos-6-2018-3
-  #   driver:
-  #     image: netmanagers/salt-2018.3-py2:centos-6
-  #     run_command: /sbin/init
 
-  ##S SALT 2017.7
+  ## SALT 2017.7
   - name: debian-8-2017-7-py2
     driver:
       image: netmanagers/salt-2017.7-py2:debian-8
@@ -81,10 +74,6 @@ platforms:
       provision_command:
         - zypper refresh && zypper install -y python-pip
       run_command: /usr/lib/systemd/systemd
-  # - name: centos-6-2017-7
-  #   driver:
-  #     image: netmanagers/salt-2017.7-py2:centos-6
-  #     run_command: /sbin/init
 
 provisioner:
   name: salt_solo