R1:c2893d86705a

Hook for database configuration plugin

Summary:
This permits individual deployments to better configure their
database configuration, e.g. to allow more dynamic configuration that reacts
to database moves or master/slave replication.

Test Plan:
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Reviewed By: epriestley
Reviewers: Girish, epriestley
CC: aran, epriestley
Differential Revision: 183