MB-12251: Use default prefix

Instead of using a random prefix for the documents, use `doc` as the
default prefix. This way every run of a test is same which makes
debugging issues way simpler.

Change-Id: Ia35ddf9ed78f160326cf7d2cbc381ae9e2712da0
Reviewed-on: http://review.couchbase.org/43854
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>