CBG-3914 change timeout to wait for authentication failure

[torcolvin]

gocb.Cluster.WaitForReady will perform the authentication handshake. If this goes slowly for some reason, it will return a timeout error instead of a gocb.ErrAuthenticationFailure

See https://jenkins.sgwdev.com/job/MasterIntegration/733/ for an example of this failure, but this fails a lot in MasterIntegration and weekly-jenkins runs.

I chose 15s as an arbitrary time. https://issues.couchbase.com/browse/MB-61174 contributes to the slowness, but this is from 7.6.0, 7.6.1 regression from 7.2 and the failure above comes from 7.2.3. I actually think we could increase this timeout.

This code behaved differently with gocouchbase buckets so I'm not sure how to 1:1 match the timeouts

sync_gateway/base/bucket_gocb.go

Line 116 in 3426f07

func GetCouchbaseBucketGoCBFromAuthenticatedCluster(cluster *gocb.Cluster, spec BucketSpec, bucketPassword string) (bucket *CouchbaseBucketGoCB, err error) {