Provide SENTRY_DSN for Notifications Center

Summary:
33fcf3f4 introduced SENTRY_DSN but it has been removed afterwards,
while Sentry wasn't currently in use.

Restore back using credential.sentry_dsn with new pillar syntax.

Ref T1791

Test Plan: Deploy container on docker-002

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1791

Differential Revision: https://devcentral.nasqueron.org/D2944