radius: T4672: Fix RADIUS server disable template logic

[bmhughes]

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4672

Component(s) name

RADIUS

Proposed changes

Change the jinja template to check for the presence of the disable dict key (rather than looking for a disabled property that doesn't exist)

How to test

Disable a RADIUS server, it will now be removed from the config file.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly