Honor --tor PATH argument

[iowaguy]

This argument to generate was previously ignored, with a hardcoded path in its
place. The hardcoded path isn't necessary at all, because the same path
is provided as a default when initializing argparse.

[robgjansen]

I think the --tor option for the tornettools generate subcommand is the path to the executable that we use during the generate process in order to generate keys for the Tor relays, not the path to the tor executable that we should use when running the Shadow simulation.

$ tornettools generate --help
...
  --tor PATH            Path to a compiled 'tor' executable, used to generate relay fingerprints. (default:
                        /home/rjansen/.local/bin/tor)
...

There is a feature request open to be able to change the default path to the tor, tgen, oniontrace, etc. executables - see #11. Likely we would need to add new options for those types of paths - either one option that would allow us to change the prefix (i.e., set something other than ~/.local/bin) or several options, one for each of the executables currently added to the shadow config file.

[robgjansen]

I don't think we will want to use the same --tor executable path for both the tornettools generate operation and for the path that we add to the shadow config file. For example, the generate and simulate steps are often run on different machines with different paths. So I'm going to close this for now, but please add discussion on #11 if you are willing to help us add support for that feature and want some feedback on the exact approach we should take there :)