rose test-battery: allow more configurable command

Change directory to `$ROSE_HOME` instead of `$ROSE_HOME/t`, and only if
current working directory is not `$ROSE_HOME`.
Allow options to `prove` to be configurable via Rose site/user
configuration.
rose test-battery: always do `prove -r`

Also updated documentation.
rose test-battery: always do `-j 9 -s`

Unless overridden by site or user configuration.
Merge pull request #1248 from matthewrmshin/improve-rose-test-battery-command

rose test-battery: allow more configurable command