tctl nodes format regression

[russjones]

In Teleport 2.4.1, the output of tctl nodes add --format=json --roles=node was a list that looked like this:

["token"]

This is becausetctl nodes add ... used to support a hidden and undocumented--count flag that could return multiple tokens.

Since then the --count flag has been removed and tctl nodes add ... always return a single token and tctl started returning something like this:

"token"

However, customers have built automation tooling around tctl that relies on tctl nodes add --format=json returning a list and we can not break users tooling. Therefore tctl should always return a list even though it only returns a single token.