[SHACK-377] Add app config values to schema

[jonsmorrow]

This change adds the new app config values for updates to the config schema so chef-run will complete without error when the user sets these values for the tray app.

[updates]
channel = 'current'
interval_minutes = 5
enable = false

Signed-off-by: Jon Morrow jmorrow@chef.io

Description

[Please describe what this change achieves]

Issues Resolved

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Check List

• [x] New functionality includes tests
• [x] All tests pass
• [x] PR title is a worthy inclusion in the CHANGELOG
• [ ] You have locally validated the change