Routers: implement POST notification for bootscript requests

[LRitzdorf]

This PR:

• Adds a generic HTTP(S) notifier function, which can be invoked to POST form data to an arbitrary endpoint
• Implements the ability to, whenever a node that requests its bootscript, POST that node's IP address to a configurable endpoint (i.e. indicating that the given node will soon require configuration)
  • The endpoint POSTed to is configurable via a CLI parameter (and associated environment variable)

This is to be used with the new TPM-manager OCHAMI component, specifically to inform the TPM manager when a new node is booting and will soon be ready to receive its TPM secret.

Implemented with help from @davidallendj

[LRitzdorf]

Also, if we could get a tagged release once this is merged, that'd be great! Mostly to facilitate Docker pulls.