Cold restart for HA environment

* There were warm restart tests so far.
It renames restart_nodes method to warm_restart_nodes
and adds real cold restart to HA test
* Move warm / cold restart tests to seperate package.
So that all new non-functional tests (for HA mode
in most cases) will be stored there.

Change-Id: I7b18129b75c52ee1ea0b80c2419f95e87667cfb7