WIP: Allow watcher to fetch configuration from server

This commit will allow HTTP and WHOIS watchers to reuse domain or url parameters from server instead of splitting configurations and duplicating watchers. I would add that group usage in network.http before this MR was unclear since url was set in watcher.

I've marked this MR as WIP before updating documentations and awaiting your remarks.

I must thanks PyCon FR 2019 that allow me to discover this project !

Before:

[testhttp-domain1]
description = "test http"
service = "network.http"
request_hostname = true
url = "https://domain1.com"

[testhttp-domain2]
description = "test http"
service = "network.http"
request_hostname = true
url = "https://domain2.com"

After:

[testhttp]
description = "test HTTP"
service = "network.HTTP"
group = "http"
request_hostname = true