Indexed Scripts/Templates : Remove routing from GetIndexedScriptRequest.

This commit removes the routing from GetIndexedScriptRequest since the script index
is a single shard that is replicated across all nodes.

Closes #7559
Indexed Scripts/Templates : Fix backward compatible template file reference

Before 1.3.0 on disk scripts could be referenced by requesting
````
_search/template

{
  "template" : "ondiskscript"
}
````

This was broken in 1.3.0 by requiring
````
{
  "template" :
  {
    "file" : "ondiskscript"
  }
}
````

This commit restores the previous behavior.

Closes #7647