Add test suite with PostgreSQL 10.0

What does this MR do?

Add a job collection to run rspec tests with PostgreSQL 10.0. Related to omnibus-gitlab#3308 (closed). Also, move MySQL tests to run only on master branch, and on schedules.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content