Fix issue requesting current EC2 role

[dspangen]

EC2 appears to have changed how you access the collection members of instance metadata--you now need a / after each collection to list them:

curl http://169.254.169.254/latest/meta-data/iam/security-credentials # nothing
curl http://169.254.169.254/latest/meta-data/iam/security-credentials/
ec2-staging-es

[remh]

Thanks a lot @dspangen do you know how it will play with older EC2 instances ?
Is that a change that got deployed only on newly created instances or to all instances ?

[dspangen]

It started breaking on instances we've had deployed for over a year.

[olivielpeau]

Thanks @dspangen for contributing! I've opened a new PR that rebases your commit and adds a trailing slash to the public-keys endpoint (#1750), so I'm going to close this one.