"if_exists" flag should apply to parent and child registers

[Rodolfo Alonso]

Some registers require the existence of a parent resource. For example, Logical_Router_Port and Logical_Router, Logical_Switch_Port and Logical_Switch, ACL and Port_Group, ACL and Logical_Switch, QoS on Logical_Switch.

The commands usually have the "if_exists" flag. If true and the resource to be modified or deleted doesn't exist, or in the case of creation commands the resource exists, the commands doesn't fail. However, in some cases this flag only applies to the parent resource but not to the child resource.

Commands where this behaviour is happening:
* Deletion of ACL in Port_Group.
* Deletion of ACL in Logical_Switch.
* Deletion of Logical_Switch_Port in Logical_Switch.
* Deletion of Logical_Router_Port in Logical_Router.
* Addition of Logical_Switch_Port on Port_Group.
* Deletion of Logical_Switch_Port on Port_Group.