Fix #221 - Add headless clustering mode

[jason-fox]

Proposed changes

• Add clustering
• Add IDM_HEADLESS ENV
• Remove GUI routes from headless.

On the basis of an ENV variable, Keyrock should be run without sessions and without a GUI just servicing the PDP and IDM and API endpoints

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• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality
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[jason-fox]

After discussion with @wistefan, we believe it makes sense to keep this functionality in a single repo under an ENV flag. The Dockerfile has been updated to also allow headless to be passed as a build ARG.