Allow setting aws_access_key and aws_secret_access_key for ebs_raid LWRP

Updated aws_ebs_raid LWRP to allow passing aws_access_key and aws_secret_access_key as parameters to the LWRP.  It was taking them from a databag.  We use encrypted databags for our AWS credentials, and would thus rather pass the keys in to the LWRP.  This is in line with the aws_ebs_volume LWRP provided by the same cookbook.