Use 'cvs rls' to check repository existance rather than checking CVSROOT/modules.

The CVSROOT/modules file is not available on every CVS server, which makes it
impossible to add some repositories. This change fixes check_repository to use
'cvs rls' instead. If this command succeeds, we know that there's an accessible
cvs server at the given cvsroot.

Testing done: Added the cvs cvs to my local server.

Fixes bug 1986.

Reviewed at https://reviews.reviewboard.org/r/5392/