Add dynamic discovery support for consul catalog based targets

[manugarg]

Consul seems to be a popular option for service registry and discovery. Adding dynamic discovery support for consul based targets to cloudprober may be useful to users across cloud platforms.

Targets config for consul will probably look something like this:

probe {
 ...
 targets {
   rds_targets {
    resource_path: "consul://service/my-service" (or /nodes)
    filter {
      key: "name"
      value: "{{.regex}}"
    }
  }
  ...
}

[manugarg]

Moved this issue to cloudprober/cloudprober#8.