add_priority_str
Description:
Adds a priority map entry into either the ingress_priority_map or the egress_priority_map properties.
The priority map maps the Linux SKB priorities to 802.1p priorities.
Parameters:
| this | the SettingVlan | 
| map | the type of priority map | 
| str | the string which contains a priority map, like "3:7" | 
Returns:
| true if the entry was successfully added to the list, or it overwrote the old value, 
            false if  |