Take into account _default_ mapping in index template when parsing alias filter in index templates.

By also taking into account the type for an index request that indexes into index that doesn't exist in the automatic create index request, the mapping for the new type will take over the `_default_` mapping. Alias filters that then point to fields in the `_default_` mapping will then not fail. 

PR for #8473
Is it still going to work if you create a type dynamically (through an indexing request) on an index that already exists?