CBQE-966: define index_key in query workload

this change makes it so that indexed_key can be defined
in query workload.  previously the template needed to
be defined and this template had indexed key.

Change-Id: I9326776f0df66c23de2538d825ce2c4fb634d706
Reviewed-on: http://review.couchbase.org/25207
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: buildbot <build@couchbase.com>