Fix module's root POST request payload

Fix module's root POST request payload

Removed “processModule” method from DefinitionGenerator class.
This method is invoked only when a specific single model is requested
directly and generated POST request had an incorrect payload.

Fixed “addRootPostLink” method to correct POST payload containing
the first container/list child.
Adapted affected unit tests.

JIRA: NETCONF-1179
Change-Id: Ic10280770cfceedc2c9c5099cdbdab14cdc28ad6
Signed-off-by: Oleksandr Zharov <oleksandr.zharov@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
(cherry picked from commit 1f6754a83ab28b43e4636c87441315614a350de2)