let loadavg be hash, then it would be extendable/overridable

the approach added in v2.0.0 is not extendable, i.e how could i set default in cookbook, and override in role or node? also current syntax allows you produce multiple "load" directives to config file which doesn't even make sense. this PR also fixes poor copy-paste from disks attributes documentation.

it should be hash based approach it's more clear. in cookbook default attributes i could have something like:

```ruby
nproc = node['cpu']['total']
node.default['snmp']['load_average'] = {
    :max1 => nproc*2,
    :max5 => nproc*1.5,
    :max15 => nproc*1.125,
}
```

and in node i override with attributes:
```json
    "snmp": {
      "load_average": {
        "max1": "20"
      }
    },
```

which would result in config file:

```
load 20 1.5 1.125
```