This allow to initially apply the same arg to all nodes in all tests.
e.g.: `test_runner.py --nodes_arg="-greatwallactivationtime=1546300800"`
will initially set all the nodes with an extra argument
`-greatwallactivationtime=1546300800`.
This option can be set multiple times to add multiple arguments.
These arguments are appended to the fixed argument list from TestNode,
but they are still overriden if they are defined by tests.