Configurable default shutdown timeouts

[pboguslawski]

Would be nice to have configuration parameters to set default shutdown timeouts per cluster (with default value equal to 120s for backward compatibility probably) and per guest; when given guest is shut down, first defined timeout should be used (in that order): from cli --shutdown-timeout parameter, from guest config, from cluster config.

It seems that default timeout is hardcoded to 120s today.

Configurable defaults would allow to protect slower guests against hard shutdowns if admin forgets to pass timeout with --shutdown-timeout cli param.