Use custom SSH keypair

For some reason I was prompted for a password when doing `vagrant ssh`
using the default insecure SSH key. And it led to me writing code to use
a custom SSH keypair. Ah well

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
README - add `--provision` flag to `vagrant up`

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>