vagrant: Increase default Virtualbox memory to 2GB
vagrant: Increase default Virtualbox memory to 2GB

Without this change, we would end up with a non-functional k8s cluster set up because of memory exhaustion.
Also made it configurable via a constant defined at top of Vagrantfile according to https://github.com/coreos/coreos-kubernetes/issues/311#issuecomment-199522832

Fixes #311

ah45/coreos-kubernetes,limres/coreos-kubernetes,chen-anders/coreos-kubernetes,turbonomic/coreos-kubernetes,bnea-engineering/coreos-kubernetes,limres/coreos-kubernetes,IDKA/coreos-kubernetes,joshix/coreos-kubernetes,amarruedo/coreos-kubernetes,ah45/coreos-kubernetes,amarruedo/coreos-kubernetes,coreos/coreos-kubernetes,amarruedo/coreos-kubernetes,ah45/coreos-kubernetes,turbonomic/coreos-kubernetes,amarruedo/coreos-kubernetes,bnea-engineering/coreos-kubernetes,chen-anders/coreos-kubernetes,coreos/coreos-kubernetes,danielfm/coreos-kubernetes,danielfm/coreos-kubernetes,turbonomic/coreos-kubernetes,limres/coreos-kubernetes,bnea-engineering/coreos-kubernetes,chen-anders/coreos-kubernetes,IDKA/coreos-kubernetes,danielfm/coreos-kubernetes,IDKA/coreos-kubernetes,joshix/coreos-kubernetes