Indexed Scripts/Templates :  Cleanup

This contains several cleanups to the indexed scripts.
Remove the unused FetchSourceContext from the Get request..
Add lang,_version,_id to the REST GET API.
Removes the routing from GetIndexedScriptRequest since the script index is a single shard that is replicated across all nodes.
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.
Remove support for preference, realtime and refresh
These parameters don't make sense anymore for indexed scripts as we always force the preference to _local and
always refresh after a Put to the indexed scripts index.

Closes #7568
Closes #7559
Closes #7647
Closes #7567