Integration tests with Vagrant and Cucumber

This commit removes the existing 'local' integration tests and replaces
them with Cucumber features running against VMs.  At this stage,
some of the assertions are pending due to the limited nature of the
response returned when executing commands through Vagrant, but the
framework is there as a starting point to build upon.

To run the suite:

    bundle exec cucumber

During development, avoid scraping the VM between runs:

    bundle exec cucumber KEEPING_RUNNING=1

Ultimately I would like to see the `TestApp` helpers along with the Vagrant
integration packaged and available for use when developing gems that work with
Cap. For now though, this closes #641
Merge pull request #664 from capistrano/feature/vagrant-cukes

Integration tests with Vagrant and Cucumber