For decomposition_multiple_db tests create the database only if it does not exist

In https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5492, we are making omnibus-gitlab create the CI database if it is specified. Right now, this conflicts with gitlab-qa's own attempt to create it, and cause it to fail

Proposal: To be backward compatible, let's first check if the database exists and attempt to create the DB if it does not exist already.