(gh-585) Add support for vagrant 1.7.x

As of version 1.7.x, vagrant will replace the default insecure keypair with a
randomly generated keypair on first 'vagrant up'. As a result the generated
ssh configs have changed and contain per-host private keys. Beaker was
breaking the path to these new private key files due to a substitution which
wasn't sufficiently constrained (the path to the private key now contains the
hostname, which was being replaced with the host's IP). This commit adds the
necessary constraint.
Merge pull request #586 from chris-reeves/gh585_vagrant_1_7_x

(gh-585) Add support for vagrant 1.7.x