Add multi routing engine support to the "routing_engine" module

[AKYD]

(this should close https://github.com/czerwonk/junos_exporter/issues/83)

• Add multi routing engine support

• Add tests

• Add _percent to metric names to make it more clear. Renamed percentage to percent ("The noun percentage requires an adjective to describe its size") :)

• Add a few extra metrics that appear on different equipment (Juniper standard are nonexistent..)

Since some info is not available on different setups, I've used the following conditional to only export the metric where the value exists. The other approach is to export it with the default value (0 for int, floats, empty string, etc). I slightly prefer to only have the metric appear when the value makes sense than always having it, but with an useless value.

if re.MemorySystemTotal > 0 {
    ch <- prometheus.MustNewConstMetric(memorySystemTotal, prometheus.GaugeValue, float64(re.MemorySystemTotal)*1024*1024, l...)
}