Add support to customize cluster ping_interval

[wallyqs]

Adds support to customize the ping_interval for routes to be different from ping_interval from clients:

port = 4222
http = 8222

server_name = 'A'

cluster {
  name = 'ABC'
  port = 6222
  routes = [
    nats://localhost:6222
    nats://localhost:6223
    nats://localhost:6224
  ]
  ping_interval = '5s'
  ping_max = 6
}

ping_interval = '30s'

[derekcollison]

LGTM