[rails_deploy_ssh_keypair_copy] Allow users to disable SSH keypair copies

Added new rails_deploy_ssh_keypair_copy parameter. This lets users
disable ssh keypair copies if they have another role that manages
the process.
Merge pull request #1 from unboxed/make-ssh-keypair-copy-optional

[rails_deploy_ssh_keypair_copy] Allow users to disable SSH keypair copies