When running the cap grid:boot command, I received the following error:

Client.InvalidKeyPair.NotFound: The key pair '/home/medined/.ec2/affy.pem' does not exist
It turned out that the EC2_KEYPAIR_NAME system variable was set incorrectly. You can find the correct value using the ec2-describe-keypairs command. Here is an example of its use:

$ ec2-describe-keypairs
KEYPAIR affy c3:e7:5f:09:50:66:f5:2b:3d:2b:ac:1d:09:52:9d:34:29:85:1b:76

So the correct value for me is affy, not the filename.