Add better DBClusterIdentifier validation (RDS::DBCluster)

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbcluster.html#cfn-rds-dbcluster-dbclusteridentifier

The DB cluster identifier. This parameter is stored as a lowercase string.

Constraints:
Must contain from 1 to 63 letters, numbers, or hyphens.
First character must be a letter.
Cannot end with a hyphen or contain two consecutive hyphens.

For additional information, see the DBClusterIdentifier parameter of the CreateDBCluster action in the Amazon Relational Database Service API Reference.