Description of problem:
--releaseversion is probably unsupported, but it should be handled gracefully, i.e. withoug throwing and exception.

Version-Release number of selected component:
dnf-1.1.1-2.fc22

Additional info:
reporter:       libreport-2.6.2
cmdline:        /usr/bin/python -OO /bin/dnf system-upgrade upgrade --allowerasing --best --releaseversion rawhide --distro-sync
executable:     /bin/dnf
kernel:         4.1.7-200.fc22.x86_64
runlevel:       N 5
type:           Python
uid:            0

Truncated backtrace:
system_upgrade.py:206:error:AttributeError: unrecognized arguments: --releaseversion rawhide

Traceback (most recent call last):
  File "/bin/dnf", line 36, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 198, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 84, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 117, in _main
    cli.configure(map(ucd, args))
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1029, in configure
    self.command.configure(self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf-plugins/system_upgrade.py", line 263, in configure
    self.opts = self.parse_args(args)
  File "/usr/lib/python2.7/site-packages/dnf-plugins/system_upgrade.py", line 254, in parse_args
    opts = p.parse_args(extargs)
  File "/usr/lib64/python2.7/argparse.py", line 1704, in parse_args
    self.error(msg % ' '.join(argv))
  File "/usr/lib/python2.7/site-packages/dnf-plugins/system_upgrade.py", line 206, in error
    raise AttributeError(message)
AttributeError: unrecognized arguments: --releaseversion rawhide

Local variables in innermost frame:
message: u'unrecognized arguments: --releaseversion rawhide'
self: PluginArgumentParser(prog='dnf system-upgrade', usage=None, description=None, version=None, formatter_class=<class 'argparse.HelpFormatter'>, conflict_handler='error', add_help=False)