Improve hostname/IP default values

Default IP+hostname values were incorrect for my VPS provider. I
improved the detection, which should give correct results results for
almost any provider. Specific issues addressed:

- icanhazip.com detection was only enabled in non-interactive mode
- `hostname` is by convention a short (non-fqdn) name in Ubuntu
- `hostname --fqdn` fails if provider does not pouplate `hosts` file
- `hostname -i` fails if provider does not populate `hosts` file
- `curl` without `--fail` will someday return crazy results
  when icanhazip.com returns 500 errors or similar
add comments to the new get_default_hostname etc. functions, and simplify the logic in the Vagrantfile and start.sh so that we always call into the same two functions
improve hostname/IP default values

Merges branch 'mkropat-populate-hostname-ip'