Add ability to set reactor_worker_threads

There is issue [1] with reactor in salt 2017, one
of the workarounds is to set reactor workers to 1

[1] https://github.com/saltstack/salt/issues/47539

Change-Id: I47d76cc1dc5d0afe6d8b215e2d32cdbab3ac1a8c
Related-Prod: https://mirantis.jira.com/browse/PROD-21463

salt/files/_reactor.conf

 {% from "salt/map.jinja" import master with context %}
 
+{%- if master.reactor_worker_threads is defined %}
+reactor_worker_threads: {{ master.reactor_worker_threads }}
+{%- endif %}
+
 reactor:
 {%- for event, reactors in master.reactor.items() %}
 - {{ event }}:

tests/pillar/master_api.sls

   master:
     command_timeout: 5
     worker_threads: 2
+    reactor_worker_threads: 2
     enabled: true
     source:
       engine: pkg