Reuse the same keypair for all the VMs of the same infrastructure

[gmolto]

When deploying a multi-node infrastructure with the IM on AWS, different keypairs are created for the different VMs of the same infrastructure. It would be convenient to create a single keypair and reuse that keypair in all the nodes of the infrastructure. This way, access via SSH to the different nodes will be much easier from the user point of view (retrieve a single private key vs retrieving N private keys for N nodes).