create slave.mesos SRV records

[lloesche]

659d1dc added the ability to resolve slave.mesos and get a list of all slave IPs. It would be nice to also get SRV records including the slave ports.

Hints from @jdef

the port shows up in two places in the slave state.json: flags.port and in pid. it's probably pretty simple to add a "pid" field to the slave struct, and then parse the port from that. sample data for pid:

slave(1)@10.2.0.5:5051
you can use mesos-go/upid to parse if you want (mesos-go is already a dep):
https://github.com/mesos/mesos-go/blob/master/upid/upid.go