VMware NSX: add sanity checks for NSX cluster backend

Ensure that all transport nodes registered are up
and running. A failure here, may prevent a lot of
failures down the lines.

Change-Id: I3473928e296ec6792f34bc27a4ae797fed337e7c
Closes-bug: #1267468
Merge "VMware NSX: add sanity checks for NSX cluster backend"