Allow setting single instance with multistage and possible bug fix?

Some small changes to make things work for our project which uses capistrano multistage as well as what I think is a bug fix for the single instance task not setting all roles.

* ec2_roles will now accept role names as well as hashes so you can setup default roles using ec2_roles
* Allow for setting load balancer as a cap variable rather than having to pass it for register_instance and deregister_instance
* Set instance name via the instance task rather than by parsing arguments so that capify_ec2 will work with multistage builds since position of server task will not be predictable
* Always add all roles for a given instance when calling the single instance task