Key Pair validation for AWS

[helgi]

Right now rigger hardcodes a keypair name deis for AWS - So does the deis.io documentation for AWS setup. I ran into issues where my fleet units under rigger wouldn't do the right thing due to the fact I had an old deis keypair floating on my AWS account since I did a setup without rigger.

Error: failed initializing SSH client: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain

This appears in a few places without much context

Comparing the local key fingerprint to the one in AWS would be good to get past obscure errors around SSH issues and inform the user to change the names (via rigger in future) or clean up their AWS account.