If you run jaeger-agent with `--reporter.grpc.host-port
dns:///jaeger-collector-hostname:14250` (note the `dns:///` part!),
the GRPC client resolves the given hostname to one or more IPs and can
load balance among the backend addresses it finds.  It even looks up
and obeys SRV records, if they exist.  This change makes jaeger-agent
use round-robin load balancing in that situation, rather than the
default of `pick_first`.  If there is only one backend, or the user
doesn't explicitly put `dns:///` before the hostname, this will
have no effect.

Signed-off-by: Benjamin Staffin <benley@gmail.com>