Skip POST for nodes without container/list child

Skip POST for nodes without container/list child

Remove generation of post operations for nodes that does
not have at least one container/list child.

Current implementation generates the wrong payload for JSON
and XML POST requests with leafs or leaf-list in the payload.

With this solution, we can avoid the need to check how many
leaves are in the container and what to do if more of them
are mandatory.

JIRA: NETCONF-1164, NETCONF-1165
Change-Id: I6268ab118fd0cb5092d4b959660855035394e436
Signed-off-by: Samuel Schneider <samuel.schneider@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>