This patch will add support for the case nsxlib is configured with
a cluster (few nsx managers), which change their availability, and
in a specific point in time might all be DOWN.
1) The nsxlib will succeed even if currently all the managers are
unavailable (state DOWN)
2) By configuration of cluster_unavailable_retry=True, when a request
is issues and all managers are DOWN, the endpoint selected will be
retried until one of the managers is back UP, or until max retries
is reached (10 by default)
Change-Id: I2e3d1a9734f37ef82859baf0082b39c11d6ce149
919909ba51