Agent: Shorten reservation ID

[mssalvatore]

Per Issue #4187, there are cases when download strings must be shorter. In order to achieve this, random strings from a set of 62 characters are generated. Using 5 characters gives something like 916M possible values, which is more than enough for any single agent.

What does this PR do?

Issue #4187.

Add any further explanations here.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Do all unit tests pass?
• Do all end-to-end tests pass?
• Any other testing performed?

  Tested by {Running the Monkey locally with relevant config/running Island/...}

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