Support the configuration of a document templates store branch

[metmajer]

Currently, the service expects a branch release/${version} to exist in the remote Git repository for the document templates store. We would like to have an environment variable that sets the expected branch name.

[clemensutschig]

@metmajer - the big question here is who wins ... suppose the call sends release/1.0 and the env var says release/preview - which version do we take..