Allow deployment artifacts to be expired

Currently artifacts linked to deployments are stored in the directory 'globalStorage' in the artifacts directory of the bamboo server.

These artifacts never expire, and so as time goes on more and more disk space is used up. In the month and a half of EBAC using deployments, we've already accumulated 25GB of deployment artifacts, and that's set to increase. It's contributing to https://extranet.atlassian.com/jira/browse/BUILDENG-3867

I'd imagine that OnDemand wouldn't be too happy with ever increase disk usage either.