Check that @config exists. Fixes #413

[calh]

This fixes #413. It depends on PR #526 only just for running the tests. The real fix is just a one-liner to check that @config exists in the database adapter. If it doesn't exist, the only side effect is that the message [Shard: name] isn't prepended to the query log.

It appears that database adapters like ibm_db and sqlserver do not use an internal config variable called @config, and the consequence is that the rails startup breaks. This quick patch at least gets people up and running.

I'm working on a more elegant fix in https://github.com/calh/octopus/tree/ibm_db, but it's not going very well. The ibm_db gem breaks on rails 5.2. I think the only two options around this would be to pull Rails 5.2 support in octopus, or try to pursue a fix in the ibm_db gem. I'd appreciate any advice on this!

[calh]

https://github.com/ibmdb/ruby-ibmdb/pull/88

Trying this from another angle too, hopefully IBM will accept my patch over there.