Add trailing slash if it doesn't exist

[bsloan-icl]

This PR adds a trailing slash to the Key Vault URL if it does not already exist. This is to fix #227 where the passphrase for SSH keys wasn't being retrieved if the URL did not contain the trailing slash.

Fixes #227

Testing done

If the Key Vault URL is missing the trailing slash, the passphrase for the SSH key is still retrieved successfully and the SSH key is created in Jenkins.

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- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[timja]

Thanks!