Introduce the vital / to find security-credentials

We can't get meta-data/iam/security-credentials, we need to get
meta-data/iam/security-credentials/ to actually get the roles
Fix unit tests to match correct meta-data security credentials URL
Merge pull request #1912 from danielgtaylor/bugfix-iam

Bug fix for IAM security credentials metadata URL. Fixes #1912, #1908, #1907.