add optional `{{source}} to ssh_config

openssh/files/ssh_config

 # Do not edit this file manually!
 # It will be overwritten by salt!
 {%- endif %}
+{%- set global_src_url = salt ['pillar.get']('__formulas:print_template_url', None) %}
+{%- set local_src_url = salt ['pillar.get']('openssh-formula:print_template_url', None) %}
+{%- if (global_src_url and local_src_url is none) or local_src_url %}
+#
+# Template used to generate this file:
+# {{ source }}
+{%- endif %}
 
 {%- if 'Hosts' in ssh_config %}
 {%- do processed_options.append('Hosts') %}