Modifies get_default_data_dir to take node id instead of enr

[mrferris]

Implements @ogenev's initial thoughts from #81.

Uses the first 8 characters of the Node ID's base 10 representation.
Any thoughts on whether we may want hex instead, or less than 8 characters?

[njgheorghita]

Personally, I'd prefer to see hex used here. Not a strong opinion, but we'd use hex to display the node id to terminal output, so it seems more consistent to also use it here.

[jacobkaufmann]

LGTM. I think @pipermerriam's comments about the --ephemeral flag in #81 can be addressed separately if desired.