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
Clean up ML2 Manager

Some things need cleanup in the ML2Manager.

1) Replace sys.exit(1) with raise SystemExit(1)
2) Replace reserved keyword type with network_type

Change-Id: I921bfaec7d3e31503942b3ca4a1b2218c44b14ac
Closes-Bug: #1259646
Merge "VMware NSX: add sanity checks for NSX cluster backend"
Merge "Clean up ML2 Manager"