Add ability to configure salt master worker threads value

Change-Id: Iff15737574a73bd0e67a38f43000f4d4de3e36e9
Related-Prod: https://mirantis.jira.com/browse/PROD-26401
diff --git a/salt/master/single.yml b/salt/master/single.yml
index 25eaf56..815c766 100644
--- a/salt/master/single.yml
+++ b/salt/master/single.yml
@@ -12,7 +12,7 @@
   salt:
     master:
       accept_policy: auto_accept
-      worker_threads: 40
+      worker_threads: ${_param:salt_master_worker_threads}
       max_open_files: 15000
       command_timeout: 10
       peer: