Mattermost updates to support providing license via existing secret outside of helm values

General MR

Summary

1. add default enterprise flag for mm - set to false
2. add existingSecret value for existing license provided outside helm values.
3. Delete enterprise enabled from test-values - this logic is now only tied to the license behavior in the chart and we do not provide a license for testing in CI.

Relevant logs/screenshots

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Linked Issue

big-bang/product/packages/mattermost#233 (closed)

Upgrade Notices

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