gazebo_ros_pkgs: use GetMaxStepSize() for the Gazebo simulation period

`GetUpdatePeriod()` will return the (desired) wall update period, not the step size in terms of simulated time. As most users will run Gazebo in real-time and this API call is only used for parameter checking, this is not a bug issue, but IMHO should be fixed anyway.

The correct API call is `GetMaxStepSize()`, which returns the increment of simulation time for each update step. The method name is somehow misleading, and from looking at the source code of [PhysicsEngine.cc](https://bitbucket.org/osrf/gazebo/src/32c3022375ccf0ccd60efe2167e9b54f8167206f/gazebo/physics/PhysicsEngine.cc) Gazebo never makes smaller steps than `GetMaxStepSize()`.