Fix: Disconnection of netconf node

Fix: Disconnection of netconf node

- Add protection to check that a node exists in TransportPCE datastores
before removing it. Appeared when the node connection remained in a
transient state, and that the node had not been created in PortMapping
and the different topologies.
- Add a new isNodeExist method in PortMapping to test presence or not of
a node in the datastore.

JIRA: TRNSPRTPCE-547
Signed-off-by: Gilles Thouenon <gilles.thouenon@orange.com>
Change-Id: I93744ea941c909c6a18e7ccc879801f83103efba