NSX: change api mapping for Service Cluster to Edge Cluster

NSX 4.2 GA has tweaked the naming for certain resources. Edge
Cluster vs Service CLuster is one of them.

Closes-bug: #1312822

Change-Id: Id4ddd99128c5e82e7a40c2d9e7385b272549cc3e
(cherry picked from commit 07a8751f799ede6b3d4868d8268148f21e17a47a)
Merge "NSX: change api mapping for Service Cluster to Edge Cluster" into stable/icehouse