(#216) Move jenkins credential logic to helpers

[steviecoaster]

Description Of Changes

This PR creates several new helper functions which cleans up the logic in the Jenkins setup step of the guide, and gives us greater control over what happens.

Motivation and Context

We encountered an edge case where the credential written to the README differed from the initial jenkins config file.

Testing

N/A at this time.

Operating Systems Testing

N/A at this time.

Change Types Made

• [x] Bug fix (non-breaking change).
• [ ] Feature / Enhancement (non-breaking change).
• [ ] Breaking change (fix or feature that could cause existing functionality to change).
• [ ] Documentation changes.
• [x] PowerShell code changes.

Change Checklist

• [ ] Requires a change to the documentation.
• [ ] Documentation has been updated.
• [ ] Tests to cover my changes, have been added.
• [ ] All new and existing tests passed?
• [ ] PowerShell code changes: PowerShell v2 compatibility checked?

Related Issue

Fixes #216

[steviecoaster]

@JPRuskin it's impossible to say what happened, but moving these out to helpers will allow support to "fix it" relatively easily.