Change the way datasources are being disabled

[radovanradic]

This is followup for https://github.com/micronaut-projects/micronaut-sql/pull/1323 and slight change how datasources can be disabled. Using global datasources.enabled was problematic because actually datasource configuration named enabled was being created in the context and when there are test-resources then it tries to resolve properties for datasource named enabled and then throws the error. This approach is maybe a bit better because each datasource can be enabled or disabled. Will need followup PR in micronaut-data to skip schema generation for disabled datasources.