feat(master): enable option to set master scheduler

pillar.example

               type: runner
               cmd: jobs.list_jobs
 
+    # Define a master scheduler
+    schedule:
+      - update_winrepo:
+        - function: winrepo.update_git_repos
+        - hours: 6
+
     # optional: these reactors will be configured on the master
     # They override reactors configured in
     # 'salt:reactors' or the old 'salt:reactor' parameters

salt/files/master.d/f_defaults.conf

 # job cache and executes the scheduler.
 {{ get_config('loop_interval', '60') }}
 
+# When using the scheduler at least one schedule needs to be
+# defined. The user running the salt master will need read access to the repo.
+{% if 'schedule' in cfg_master -%}
+{%- do default_keys.append('schedule') %}
+schedule:
+{%- for schedule in cfg_master['schedule'] %}
+{%- if schedule is iterable and schedule is not string %}
+  {%- for name, children in schedule.items() %}
+    {{ name }}:
+  {%- for child in children %}
+    {%- for key, value in child.items() %}
+      {{ key }}: {{ value }}
+    {%- endfor -%}
+  {%- endfor -%}
+  {%- endfor -%}
+{%- endif -%}
+{%- endfor -%}
+{%- endif %}
+
 # Set the default outputter used by the salt command. The default is "nested".
 {{ get_config('output', 'nested') }}