ofproto-dpif-xlate: Change priority tags from boolean to enum

ofproto-dpif-xlate: Change priority tags from boolean to enum

Priority tags is a port configuration to determine how the port treats
priority tags, e.g. zero VLAN ID. Change the type from boolean to enum
as a pre-step towards introducing additional modes. The new options are
"never", equivalent to previously "false", and "if-nonzero",
equivalent to previously "true". "true" is still supported for backwards
compatibility.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>