Add 'scl' builder option for software collection name

With a general 'scl' builder option, a software collection name can be
specified irrespective of the releaser used.  RsyncReleaser's own 'scl'
option has been deprecated in favour of using 'builder.scl' in
releasers.conf.
Merge pull request #122 from domcleal/scl-builder-arg

Add 'scl' builder option for software collection name