Replace #getPath in processInstanceIdentifierType

Replace #getPath in processInstanceIdentifierType

DefinitionGenerator#processInstanceIdentifierType method was used to
create an example of the instance-identifier leaf node's value.
For this purpose SchemaPath#getPath method was used to search for 'root'
container.

In fact, this approach leads to incorrect results because it can for example
find an RPC definition as 'root' container. RPC definition cannot be used as
instance-identifier value because its not a data node.

We have reworked this method to use as an example the first container child
of the node's model.

This way we can also get-rid of deprecated SchemaPath#getPath method.

Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Change-Id: I2a4f1dc2e76ae9117bc826e45b0d9c0379cb5213