Exempt some tables from sharding key requirements

Some tables we have today can be exempted from having a sharding key.

For example:

• JiHu specific tables have no data on the gitlab.com database, and hence can be exempted from the requirement of having a sharding key or desired sharding key set.

Implementation plan:

• Introduce a keyword in the yml like exempted_from_sharding: true, to identify such tables are exempted.
• Such exempted tables should be avoided from being counted in our dashboards.