deploy: support for rsync module paths

Current deployment code did not support for rsync modules, which defined like username@example.com::fdroid. https://gitlab.com/fdroid/fdroidserver/-/blob/50aa35772b058e76b950c01e16019c072c191b73/fdroidserver/deploy.py#L752-759

What about adding support for it? Easiest way to add it:

        s = serverwebroot.rstrip('/').split(':')
        if len(s) == 1:
            fdroiddir = s[0]
        elif len(s) == 2:
            host, fdroiddir = s
        elif len(s) == 3 and len(s[1]) == 0: // space between two colons must be empty
            host, fdroiddir = s[0], s[2]
        else:
            logging.error(_('Malformed serverwebroot line:') + ' ' + serverwebroot)
            sys.exit(1)