chore(contrib/vagrant): update vagrant box to coreos-295.0.0
Merge pull request #796 from deis/coreos-295.0.0

chore(contrib/vagrant): update vagrant box to coreos-295.0.0
refactor(Vagrantfile): merge Vagrantfiles and update to upstream

This commit merges the Makefiles and Vagrantfiles, and supports
both a 1-node and n-node local cluster with the same files. Also,
the user-data and override-plugin have been updated to be more
closely in line with upstream. Also updates README with additional
instructions.

closes #760, #747