Provide an option to randomize domain name

This commit lets users enable hostname randomization from their
Vagrantfile. Without this option enabled, domain creation will fail when
multiple VMs are spun up from the same Vagrantfile, due to a domain name
conflict.
Update readme and test cases

Updated documentation so that users could find the new option.

Updated test case to behave as expected.
Now with less with
Merge pull request #216 from nicot/master

Add random hostname option