MB-6257: default to main REST user/pswd during ddoc restore
MB-6257: default to main REST user/pswd during ddoc restore

If the couchApiBase doesn't provide an override REST user/pswd,
default to use the main REST user/pswd when restoring a design doc.
This is especially important for SASL buckets that have their own
password.

Change-Id: I02dff821500b1a200e5c155c33ca601dcc85a192
Reviewed-on: http://review.couchbase.org/19908
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

couchbaselabs/couchbase-cli,couchbase/couchbase-cli,membase/membase-cli,membase/membase-cli,couchbaselabs/couchbase-cli,couchbase/couchbase-cli,couchbaselabs/couchbase-cli,membase/membase-cli