[CX] Hostname:port specification for external links is not flexible

Currently, external links hostnames must be specific to the transport. e.g. if "full" encryption is requested, the supplied hostname must be hostname:httpsport, hostname:httpport (or just hostname) for other encryption types. XDCR configuration is more flexible, and attempts to determine the correct port for the configured security settings.

Configuring external links should be improved to calculate the correct port when able.