Definition export: change user tags to a JSON array

[michaelklishin]

Proposed Changes

This makes exported user definitions generate the user.tags property as an array of strings instead of a comma-separated list in a single string. This makes significantly more sense in JSON. This is also a breaking change
to a behavior that's some nine years old. In order to limit the effects of this, we support both formats when
importing user definitions.

This means that

• Starting with this PR, nodes will be able to import definitions in both original and updated formats
• Older nodes won't be able to import definitions in this format (newer nodes), which we believe is a reasonable limitation

Types of Changes

• Bug fix (non-breaking change which fixes issue Export user tags as a list #2667)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

HTTP API clients such as Hop, Rabbit Hole, and the Ruby one will have to be updated.

Closes #2667.