Index Template: Parse and validate mappings in template when it puts

Parse and validate mappings in template when it puts.

* parse and validate mappings in template using dummy index service
* return error if template include invalid mapping 
* Not support a partial template

Close #2415
I see and I agree your concern...
I think two plans for validating index templates.

1. Validating mappings without IndexService
First, I try to develop without IndexService.
However, it is difficult to validate without IndexService.
DocumentParser depends on IndexSettings. 
And to validate, we should parse the mappings.

2. Creating dummy index
It is this PR.
If we can create temporary/dummy index, that means without persist index, it is useful for this issue.

I re-try to use only DocumentMapperParser.