Adds stop, switchConfig, and start atrt commands that can be called from
tests.

These commands allow support of downgrade tests in which we are required
to stop all management nodes, and then start them one at the time with
new configuration (different procedure than rolling restart).

Rolling restarts can still be done with the previously existent command
"change_version". Additionally, the stop and start commands can be use
for any other test scenarios that requiring stopping and/or starting a
data node.

Minor formatting issues in AtrtClient.cpp are also fixed.