Add standard SSL options to mysql input (#2933)

plugins/inputs/mysql/README.md

@@ -81,6 +81,11 @@ This plugin gathers the statistic data from MySQL server
   #
   ## Some queries we may want to run less often (such as SHOW GLOBAL VARIABLES)
   interval_slow                             = "30m"
+  
+  ## Optional SSL Config (will be used if tls=custom parameter specified in server uri)
+  ssl_ca = "/etc/telegraf/ca.pem"
+  ssl_cert = "/etc/telegraf/cert.pem"
+  ssl_key = "/etc/telegraf/key.pem"
 ```
 
 ## Measurements & Fields