new flags for ground_motion_correlation_params

For a GMF calculation with correlation:
In OpenQuake v1.0, the syntax was: ground_motion_correlation_params =
{"vs30_clustering": false};
In OpenQuake v1.2, the syntax is: ground_motion_correlation_params =
{"vs30_clustering": False} with capital F; the same for true, now it
must be written True; 0 and 1 are also acceptable flags in v1.2