Jonathan/cascache cell var from loop

Description

This MR fixes the request_stream function in https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/_artifactcache/cascache.py#L333 raising a flag in pylint (cell-var-from-loop).

Changes proposed in this merge request:

• Instead of reading resource_name from its containing scope, it's read as an argument

This merge request has no associated issue, and was first discussed in https://irclogs.baserock.org/buildstream/latest.log.html#t2018-08-20T14:38:12