Expose a way to fully customize metadata source configuration

[detiber]

Currently the metadata server is determined using the value of TINKERBELL_IP from the environment, but we should also likely allow configuration of the port, protocol or even the full URL:

Originally posted by @displague in #100 (comment)

[chrisdoherty4]

tinkerbell/proposals#61 would make this more or less superfluous so its on hold until that proposal moves.