Accept multiple roles

Sometimes, it is useful to have a server which can be tagged with multiple roles, and for Capistrano to recognize it in those roles. The Capistrano configuration is locked down, so it's rather difficult to check if a server is already in the host set, so instead I just modified `get_servers_for_role` to accept multiple roles.

`ec2_role [:web, :worker]` will find servers either either or both roles, and tag them appropriately. Before this change, the server may only possess one role or the other.