indexer: Generate index file path using system config value

Indexer will store index files in storage directory specified
by the system config, indexer.storage_dir.

Change-Id: If16e5c06c3c38b55d7ba7631612295e38eda944a
Reviewed-on: http://review.couchbase.org/44173
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>