Referencing stored scripts by id does not work inside watcher actions

[andrershov]

Elasticsearch version (bin/elasticsearch --version): 6.5.0

Plugins installed: none

JVM version (java -version):

java version "11" 2018-09-25
Java(TM) SE Runtime Environment 18.9 (build 11+28)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11+28, mixed mode)

Description of the problem including expected versus actual behavior:
Referencing stored scripts by id does not work inside watcher actions.

Steps to reproduce:

1. Create a stored script

{
  "script": {
    "lang": "mustache",
    "source": "executed at {{ctx.execution_time}}"
  }
}

2. Create a simple watcher with log action referencing this script:

{
  "trigger" : {
    "schedule" : { "interval" : "10s" }
  },
  "input" : {
    "http" : {
      "request" : {
        "host" : "localhost",
        "port" : 9200,
        "path" : "/_cluster/health"
      }
    }
  },
  "actions" : {
  "log" : { 
    "logging" : {
      "text" : {
    	 "id" : "log-action"
      }
      }
    }
  }
}

In logs you will see:

[2019-03-19T17:11:06,919][INFO ][o.e.x.w.a.l.ExecutableLoggingAction] [eQ3F5fy] log-action

instead of:

[2019-03-19T17:09:05,768][INFO ][o.e.x.w.a.l.ExecutableLoggingAction] [jR_IkMm] executed at 2019-03-19T16:09:05.739Z

This is also reproducible for other actions, such as e-mail.
The issue is not repoducible in Elasticsearch version 6.4.
The cause of the issue is most likely the following PR #33978.

In particular, this fast path triggers

int indexStartMustacheExpression = template.indexOf("{{");
if (indexStartMustacheExpression == -1) {
            return template;
 }

which treats log-action as a template.

[elasticmachine]

Pinging @elastic/es-core-features