Updated release_public_ips and boot_instances

Comparing addr.instance_id with null string to verify available
address.

Added 4 second timeout in boot_instances() while checking for
instance status.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>
Updated security group removal in cluster.delete()

Signed-off-by: Shashank Sahni <shredder12@gmail.com>