New Datasource: circonus_broker

[radeksimko]

User shouldn't have to look up numeric ID of a broker and codify it in the config, e.g. in circonus_check.

It would be great to have a data source which can look up broker's ID, so the user can reference it like this:

data "circonus_broker" "london" {
  filter {
    key = "cn"
    value = "uk1-1.noit.circonus.net"
  }
}

resource "circonus_check" "consul_server" {
  active = true
  name = "radek-test"
  period = "60s"

  collector {
    id = "${data.circonus_broker.london.id}"
  }
...