If connection-config has no roots at all, return error

[tock-ibm]

// ConnectionConfig required configuration in order to
// open session with BCDB instance, replica set informations
// servers root CAs
type ConnectionConfig struct {
// List of replicas URIs client can connect to
ReplicaSet []*Replica
// Keeps path to the server's root CA
RootCAs []string
// Logger instance, if nil an internal logger is created
Logger *logger.SugarLogger
}

when creating a db instance in bcdb.Create, check that there is

• at least one CA cert
• at least one replica