rose config-dump: fix -C DIR and -f FILE behaviour.

Only process files given in the list of -f FILE options. If -C DIR is
specified with -f FILE and FILE is a relative path, FILE is assumed to
be relative to DIR.
rose config-dump: test absolute "-f FILE".
Merge pull request #688 from matthewrmshin/684.fix.rose-config-dump--file

rose config-dump --file=FILE behaviour fix

Closes #684.
Merge branch 'master' of github.com:metomi/rose into gh-pages