More attribute types

http://help-ja.mackerel.io/entry/spec/agent#config-file

```
pidfile = "/path/to/pidfile"
root = "/path/to/mackerel-agent-root"
verbose = false
roles = [ "My-Service: app" ]

[sensu.checks.vmstat]
command = "ruby /etc/sensu/plugins/system/vmstat-metrics.rb"
type = "metric"
```

Now only support apikey and apibase.