Hi @syamgk,
Mattermost Team Edition is intended for self-hosted team communication, it's not intended to be used at a scale where you'd need a database cluster. To officially support such a configuration would be too much of a tax on the open source community to setup such an environment and to test, document and support it properly, even if a change was submitted.
There's a commercial extension, Mattermost Enterprise Edition, intended for large organizations that supports DB clusters (along with advanced access controls, compliance reporting, corporate directory integration and other features needed for advanced organizations beyond what teams need).
If you're at an enterprise needing secure, modern messaging, it's an excellent value. If you're a non-profit or academic institution there's steep discounts available.
You're also welcome to fork the open source version and modify it as you wish--and many organizations do.
DB clustering is just out-of-scope for the Team Edition.