PCBC-186: Add method to list all design docs

Change-Id: Idd2857ea741dfee54dab3cdb86e02fac88220934
Reviewed-on: http://review.couchbase.org/25441
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>