config: rename metrics -> plugins

metrics are the results, plugins are the ones that get the metrics
metrics: fix PluginState constructor
metrics: standardize metrics

A metric is composed of the following data:

[instance1 -> {metric1: value1, metric2: value2},
 instance2 -> {metric1: value3, metric2: value4}]

For example, a metric for diskusage can be:
['sda1' : {'usage': 80%},
 'sda2' : {'usage': 50%}]

This way we can cover plugins that measure multiple instances
at the same time, like disk usage. And also plugins that generate
multiple different metric entries, like cpu usage that shows the
current value and the previous X mins value
metrics: load plugin thresholds from config