Customize node exporter target

[arbulu89]

Description

Add a new configuration option to give the user the option to choose the node expoter target.

The new parameter is node-exporter-target in the command line and configuration file.
If the option is not used, the code will use the lowest ipv4 address plus the default node exporter port.

And it will send a new field in the host discovery as prometheus_targets with a map[string]string with all the configured exporters.
I have chosen this option to enable future exporters usage. In this case:

{
...
    "prometheus_targets": {
        "node_exporter": "10.1.2.3:9100"
    }
}

How was this tested?

UT

Did you update the documentation?

I added some "in code" documentation, but we will need to add this new value in additional places I guess

[nelsonkopliku]

LGTM