feat(kitchen): use workaround to prevent opensuse-leap-15 SCP failures

* Workaround to avoid intermittent failures on `opensuse-leap-15`:
  - SCP did not finish successfully (255):  (Net::SCP::Error)
* Based on:
  - https://stackoverflow.com/a/47880689

ssf/defaults.yaml

@@ -14,7 +14,8 @@ ssf_node_anchors:
             #       release phase via. `pre-commit_semantic-release.sh`
             #       An alternative method could be to use:
             #       `git describe --abbrev=0 --tags`
-            title: 'ci(travis): use `dist: bionic`'
+            # yamllint disable-line rule:line-length
+            title: 'ci: use `dist: bionic` & apply `opensuse-leap-15` SCP error workaround'
             body: '* Automated using https://github.com/myii/ssf-formula/pull/22'
           github:
             owner: saltstack-formulas

ssf/files/default/kitchen.yml

@@ -135,6 +135,12 @@ platforms:
       {%- elif os == 'centos' and os_ver == 6 %}
       run_command: /sbin/init
       {%- endif %}
+    {%- if [os, os_ver] == ['opensuse/leap', 15] %}
+    # Workaround to avoid intermittent failures on `opensuse-leap-15`:
+    # => SCP did not finish successfully (255):  (Net::SCP::Error)
+    transport:
+      max_ssh_sessions: 1
+    {%- endif %}
 {%-   endif %}
 {%- endfor %}