Add support for default Cassandra port

[snicoll]

Follow-up of #18621

In Spring Boot 2.2.x, Cassandra has a port defaulting to 9042 and contact-points as a list of host, defaulting to localhost. In Spring Boot 2.3, the port is no longer necessary as all contact point must be of the form host:port.

There is a case to be made to perhaps massage the input configuration a bit and automatically translates user's config to the expected pattern, including when a custom port is set. The only downside is that the default value of contact-points would be a bit odd if we accept contact points with the form host.

[snicoll]

We've decided to bring the port property back and use it if a contact-point does not have a specific port. The description of the property (and contact-points) should be adapted accordingly.

[snicoll]

We should review #19779 (comment)