rose app-run: rose_prune.

Built-in application to housekeep a cycling suite.
rose_prune: document and debug.
t:rose-task-run: basic tests for rose_prune.
t: simplify settings for optional tests.

Single set of configurations to determine whether to run tests involving
remote job hosts.
Merge branch 'master' into housekeep
rose test-battery: default option `-j 6 -r`.
Merge pull request #761 from matthewrmshin/housekeep

rose_prune: built-in application to housekeep a cycling suite