rose.config.dump: use temporary file to stage

If process is killed or if storage is full, the worst we'd have is a
partially written temporary file in the same file system, instead of a
partially written target file.
rose bush: recognise the `ready` status

This change is in anticipation of cylc/cylc#794.
Merge pull request #1068 from matthewrmshin/submitting-to-ready

rose bush: recognise the `ready` status
Merge pull request #1067 from matthewrmshin/rose.config.dump.temp

rose.config.dump: use temporary file to stage