use ietf client/server models for configuring call home feature

The call home feature uses ad hoc configuration models (see the associated
yang file). We should move to using a "standard" model where we have the
choice.

The IETF models are at

https://tools.ietf.org/html/draft-ietf-netconf-netconf-client-server-01

This will be an API change, but the call home feature should be marked
as experimental.

The call home feature (as of this writing) is at this patch
https://git.opendaylight.org/gerrit/#/c/49253/8