auto-create EC2 key pair

[coyotemarin]

Looks like we should be able to automatically create key pairs through the EC2 API so that SSH will always work.

Some things to consider:

• should be a way to specify no key pair (maybe by setting it to '')
• having an auto-created key pair shouldn't keep you from joining a pooled job flow with a different key (there should just be a warning that explains why you can't connect to it).
• should be able to specify ec2_endpoint, I guess?

[coyotemarin]

Also, do we always store the auto-created key in ~/.ssh/mrjob/?

[coyotemarin]

And of course this shouldn't fail for users who don't have permissions to create keys.