Transport: Remove parsing of port ranges

UnicastZenDiscovery allowed you to specify port ranges for hosts to connect to.
However only the first port was used at all, when specifying something like
`host[9300-9400]`. This commit only accepts `host:port` pairs and thus makes
the configuration reflect the execution as even in the old setup only a single
host:port pair was pinged at all, as this was very expensive.

Note there is a coming PR for 1.x, which deprecates this setting and emits a warning instead of removing it.
We talked with @kimchy a while ago about increasing LIMIT_PORTS_COUNT to 2.
See thread here: https://github.com/elasticsearch/elasticsearch-cloud-aws/issues/99#issuecomment-47430809