Do not depend on machine.config.vm.hostname

Hello.

landrush currently depends on `config.vm.hostname = "something"` to be defined in Vagrantfile. However, vagrant does not support changing hostname on some systems like TinyCore (I'm using boot2docker, which uses TinyCore).

This pull request looks for the config value. If it's not available, it reads the host name from the system. If in some edge case it is not defined, it default to "guest-vm".

What do you think?