Properly handle template branch names that contain a forward slash

If the `templateBranchName` is a branch containing a forward slash (e.g. `dev/v1.0`) this leads to errors when trying to retrieve the job's config from Jenkins (since Jenkins interprets the `/` as part of the URL and therefore throws a 404). This PR runs the `templateBranchName` through the same conversion (`/` -> `_`) as the job names created from git branches. Thus, for a `templateBranchName` like the above example, the expected name of the template **job** is `dev_v1.0`, matching the general pattern the script expects.