feat(Vagrantfile): add serial console output sections
fix(serial): Use absolute paths for serial log files
refactor(system): Replace system() with FileUtils calls
fix(plugin): Remove /etc/environment hack
README: point out you need new vagrant
fix(serial): Disable serial logging by default

The serial console logging causes VirtualBox to consume an immense
amount of CPU, making coreos-vagrant nearly unusable. Disabling it
by default treats the problem until VirtualBox can fix the problem
upstream.
feat(config): Use config.rb for configuration
added config variables CORE_FOLDER and HOST_FOLDER
changed default NUM_INSTANCES from 1 to count of vagrant machines in this directory
oops! handle new directory (zero machines created)
moved config variabes to config.rb