MB-11962: Use any hostinfo to retrieve design docs

When an external hostname is used to do backup, the tool won't find
it from internal server list. However, it is ok to run request against
any host in the cluster to get design docs.

Change-Id: If3ec34025e98b1b7e88eba95cd5e306005263016
Reviewed-on: http://review.couchbase.org/40666
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>