bitbucket-pipe-release to push latest tag on configurable branch name

Currently the code is hardcoded to only tag latest if we're on branch `master`

We have moved away from this naming convention however in our pipe repos we have to keep `master` if we don't want to add any additional logic to have our docker image tagged latest.

here's the current code used - https://bitbucket.org/atlassian/bitbucket-pipe-release/src/master/pipe/pipe.py

if branch == 'master':            
   return ['latest', version]