Minimal "lifetime" of transient cluster

Option "min_transient_cluster_active_time" to Savanna configuration file was added.
This option allows to keep alive a transient cluster within "min_transient_cluster_active_time" timeout.

Partially Implements: blueprint trust-support-in-i-tests

Change-Id: Id37f4054bf0adf098c917874a713153ac15fe1ec
Merge "Minimal "lifetime" of transient cluster"