Improve entity portability when using YFilter

[111pontes]

The use of YFilter makes entities non-portable when used with the CRUD service. Filters may need to be defined as NETCONF filters with the understanding that when applied to an entity, they make it non-portable across different providers. The current filter semantic is NETCONF specific, not CRUD specific.

[ygorelik]

During gNMI Service feature development the YFilter enum has been expanded to include gNMI specific filter - update. Therefore the enum is used for both Netconf and gNMI services. In order to assure portability the filter values, which are not applicable for the specific service can be internally substituted to corresponding once. For example: the gNMI update can be internally substituted by merge for Netconf.