Add flag to choose advertising local IP to Prysm node

This PR addresses #31

Changes work locally:

with the flag on the command:

level=INFO msg="Adding ourselves as a trusted peer to Prysm" peer_id=16Uiu2HAmBpAB8GPWRTC77i8ZUqJSoqTatiaPqqcJDrPGMYwhXM1M "on local maddr"=/ip4/127.0.0.1/tcp/9020

without the flag on the command, and the default behaviour:

level=INFO msg="Adding ourselves as a trusted peer to Prysm" peer_id=16Uiu2HAmTTJTsbhNvWAwjEBck21g3Ap4FFBUnDxnTFPzavzdWHRQ "on priv maddr"=/ip4/192.168.1.109/tcp/9020

probe-lab / hermes

Add flag to choose advertising local IP to Prysm node #33