Adds a 'ssh_keypair_name' option to the configuration
for AWS, along with some munging to create the
temporarily keypair if one isn't specific.

NOT YET WORKING.

From a 'make' I get the following errors:

builder/amazon/ebs/builder.go:94: b.config.SSHKeyPairName undefined
(type config has no field or method SSHKeyPairName)

builder/amazon/instance/builder.go:199: b.config.SSHKeyPairName
undefined (type Config has no field or method SSHKeyPairName)