ovn-sbctl: support setting rbac role for remote connections

ovn-sbctl: support setting rbac role for remote connections

Add support for specifying rbac "role" when setting remote
connection configuration in the southbound database.

Prior to this change, usage examples included:

    ovn-sbctl set-connection ptcp:6642
    ovn-sbctl set-connection pssl:6642 \
                             read-only ptcp:7777 \
                             read-write punix:/tmp.foo

With this change, in addition to the above:

    ovn-sbctl set-connection role=ovn-controller pssl:6642 \
                             read-only role= ptcp:7777 \
                             read-write punix:/tmp/foo

As with the "read-only"/"read-write" attributes, the specified
role is applied to all subsequent connections until changed.

Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>