drive libp2p behaviour identity from configuration to indicate network type

[ryardley]

https://github.com/gnosisguild/enclave/blob/main/crates/net/src/net_interface.rs#L169

This aught to eventually be driven by a flag we can just hardcode for now but it probably should indicate how it should be used as a network identifier:

/enclave/testnet/v1 maybe?