Add parameter for setting port used in the Cassandra Module

We should add the port used when connecting to a Cassandra cluster. Right now in both files (gora-mapping and gora-properties) we have to set the server name along with the port, but this is sometimes confusing for users.
gora.cassandrastore.servers=myserver:9160
So adding:
gora.cassandrastore.port
and:
PORT
inside the mapping file.