Add role change command support

When ryu-rfproxy is passed the --ofp-role commandline argument, it will
advertise this role to all datapaths that connect. Valid values are
'master', 'slave' or 'equal'. If in 'slave' mode, routemods will not be
forwarded to datapaths.

Signed-off-by: Srijan Mishra <srijan23mishra@gmail.com>
Signed-off-by: Joe Stringer <joestringernz@gmail.com>