[ADDED] URLs to cluster{} in /varz and update of gateway ones

[kozlovic]

In varz's cluster{} section, there was no URLs field. This PR adds
it and displays the routes defined in the cluster{} config section.
The value gets updated should there be a config reload following
addition/removal of an url from "routes".

If config had 1 route to "nats://127.0.0.1:1234", here is what
it would look like now:

"cluster": {
    "addr": "0.0.0.0",
    "cluster_port": 6222,
    "auth_timeout": 1,
    "urls": [
      "127.0.0.1:1234"
    ]
  },

Adding route to "127.0.0.1:4567" and doing config reload:

"cluster": {
    "addr": "0.0.0.0",
    "cluster_port": 6222,
    "auth_timeout": 1,
    "urls": [
      "127.0.0.1:1234",
      "127.0.0.1:4567"
    ]
  },

Note that due to how we handle discovered servers in the cluster,
new urls dynamically discovered will not show in above output.
This could be done, but would need some changes in how we store
things (actually in this case, new urls are not stored, just
attempted to be connected. Once they connect, they would be visible
in /routez).

For gateways, however, this PR displays the combination of the
URLs defined in config and the ones that are discovered after
a connection is made to a give cluster. So say cluster A has a single
url to one server in cluster B, when connecting to that server,
the server on A will get the list of the gateway URLs that one
can connect to, and these will be reflected in /varz. So this is
a different behavior that for routes. As explained above, we could
harmonize the behavior in a future PR.

Signed-off-by: Ivan Kozlovic [email protected]

[derekcollison]

LGTM

[derekcollison]

LGTM