pkg/rhcos/ami: Update name filter to "redhat-coreos-*"

The OS folks have a new pipeline for publishing these AMIs, and
they're setting these new names.  Current output:

  $ AWS_DEFAULT_REGION=us-east-1 aws ec2 describe-images --image-ids ami-0fd0205c3c81d0412 --output json
  {
     "Images": [
         {
             "VirtualizationType": "hvm",
             "Description": "Red Hat CoreOS \"maipo\" (OpenShift) 47.33",
             "Tags": [
                 {
                     "Value": "redhat-coreos-maipo-47.33-hvm",
                     "Key": "Name"
                 }
             ],
             "Hypervisor": "xen",
             "EnaSupport": true,
             "SriovNetSupport": "simple",
             "ImageId": "ami-0fd0205c3c81d0412",
             "State": "available",
             "BlockDeviceMappings": [
                 {
                     "DeviceName": "/dev/xvda",
                     "Ebs": {
                         "Encrypted": false,
                         "DeleteOnTermination": true,
                         "VolumeType": "gp2",
                         "VolumeSize": 8,
                         "SnapshotId": "snap-08318a8024617da0b"
                     }
                 },
                 {
                     "DeviceName": "/dev/xvdb",
                     "VirtualName": "ephemeral0"
                 }
             ],
             "Architecture": "x86_64",
             "ImageLocation": "531415883065/redhat-coreos-maipo-47.33-hvm",
             "RootDeviceType": "ebs",
             "OwnerId": "531415883065",
             "RootDeviceName": "/dev/xvda",
             "CreationDate": "2018-10-30T15:27:07.000Z",
             "Public": false,
             "ImageType": "machine",
             "Name": "redhat-coreos-maipo-47.33-hvm"
         }
     ]
  }

For the previous pipeline, see the output in d01ac5d (pkg/rhcos:
Default to the most-recent AMI, 2018-09-19, #290).

pkg/rhcos/ami.go

@@ -34,7 +34,7 @@ func AMI(ctx context.Context, channel, region string) (ami string, err error) {
 		Filters: []*ec2.Filter{
 			{
 				Name:   aws.String("name"),
-				Values: aws.StringSlice([]string{"rhcos*"}),
+				Values: aws.StringSlice([]string{"redhat-coreos-*"}),
 			},
 			{
 				Name:   aws.String("architecture"),