Changes for local multinode

[cytadela8]

We should make it possible to launch multiple local nodes without conflicts by changing environment variables. To do that we have to make it possible to change following configs with environment variables:

• persistence table path
• node port

[thepiwo]

Yes we could make the port phoenix listens on, configurable via environment variable

[thepiwo]

@meivantodorov can you check why the persistance breaks when starting multiple nodes? I tried with PORT=4001 iex -S mix phx.server and PORT=4002 iex -S mix phx.server in a different terminal