[JENKINS-65530] Add dbIndex support for Redis.

[cafuego]

This PR is a naive attempt at adding redis dbindex support. There is a new text field on the advanced config screen to enter the dbindex (defaults to 0) and if the dbindex is > 0. Jedis will call .select(dbindex) before trying to send data.

The changes did not break tests, and appear to work as expected on my install. Which is good? :-)

https://issues.jenkins.io/browse/JENKINS-65530

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