Before this change, keepalive probe consisted of two separate
configurable roundrip - one based on keepalive_section attribute,
and one on validation_method.
The recommended way to probe NSX appliance is using node/health API,
and tests show that it has best roundtrip time. This nsxlib will
switch to this healthcheck, and not expose keepalive methology to
clients any longer.
Change-Id: Ia972ef3d087fd01fa18d5a4e9dc9c32fbed0eb40
69db9a9167