InfluxDBOut can't create a new retention policy when the database already exists

InfluxDBOut always issues the `CREATE DATABASE dbname WITH NAME rpname` query, which fails if the database already exists but the retention policy does not.

Start by tracing InfluxDBOut's handling of the `CREATE DATABASE dbname WITH NAME rpname` query and identify the tests covering database and retention-policy creation. Reproduce the case where the database exists but the retention policy does not, then verify that the retention policy can be created without failing on the existing database.

Written by the indexing model from the issue text.