Added support for rds with a non default port. Mainly used while runn…

[shahalh]

…ing tests with more than one Localstack container on the same machine, to avoid port allocation issues.

[shahalh]

@whummer Thank you for you reply :-) Our use case is for running multiple Localstack instances on the same server, so control over the exact port per instance is critical in order to avoid port binding issues. I'm not sure I understand why only exposing the port range is sufficient. Would you happen to know if we would be able to build and use our custom version? When I ran the mvn install, I was missing keys for signing and the install phase failed.