Depend on information_schema for MyXQL driver

[HammamSamara]

This pull request remove the dependency on the tenants table in favor of the existing information_schema.schemata table, which brings MySQL tenancy creation flow closer to the postgres flow.

[coveralls]

Coverage increased (+0.01%) to 99.515% when pulling 41ff67a on Userpilot:drop-tenants-table into bdf19bb on ateliware:master.

[kelvinst]

Sorry it took me so long to review, I left you a comment, and I still need to test it manually on my side, to be sure backwards compatibility is maintained.

[HammamSamara]

Sorry it took me so long to review, I left you a comment, and I still need to test it manually on my side, to be sure backwards compatibility is maintained.

Thank you for taking the time reviewing the change set. As you may anticipate it, getting rid of the tenants table might be a breaking change for some that needs to be reflected on the repo mix versioning.

[kelvinst]

Yeah, but I think we could make it backward compatible by rollbacking the tenants_table config and using the old code for when the config exists. So we should just change the default to information_schema and notify users that when upgrading, they should either run a migration to send all the tenants from the tenants table to the information_schema or set the tenants_table config and keep using the same as before.

[HammamSamara]

Rolling back most of the changes to keep alignment with the tenants table, while using information_schema as the default config.
Users need to set tenants_table as :tenants like I do for tests to keep it backward compatible:

config :triplex, tenant_table: :tenants

[HammamSamara]

After a quick thought about compatibility and for safer merge, I am setting the default behavior to work with the :tenants table so all users segments don't need to change anything now (100% backward compatible).

If anyone (like me) wishes to not work with :tenant table and generates the associate migrations, then they can just configure Triplex to work with the "information_schema.schemata" as mentioned in the docs.

config :triplex, tenant_table: :"information_schema.schemata"

[kelvinst]

Pretty cool! Awesome work, thanks! And sorry for all the back and forth. Merging it to master, will release a new version of triplex with your changes soon.

[kelvinst]

@HammamSamara one last request: could you open a new PR changing the default behavior to use the information schema? I think that's a sane default as it's simpler than having to add a new table. We will then release that other one on 2.0

[HammamSamara]

PR opened here #81