Allow for creating a list of AWS EC2 key pairs

[gregharvey]

The current role supports creating a single key pair, it would be nice to be able to provide a list. For backwards compatibility it should still support a string and convert it to a list with one item, for example:

• https://github.com/codeenigma/ce-provision/blob/2.x/roles/debian/ssl/tasks/main.yml#L10-L13