CBQE-0: n1ql: do not create primary index if already exists

Change-Id: Ia6fdce640c474352f67686789dad54260184da65
Reviewed-on: http://review.couchbase.org/41642
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>