Launch parameter errors in gzserver.launch.py

[scpeters]

I'm seeing some weird console errors using the ros2 branch with the following launch command:

$ ros2 launch gazebo_ros gzserver.launch.py verbose:=true physics:=ode
...
[Err] [Server.cc:472] Could not open file[]
[Wrn] [Server.cc:381] Falling back on worlds/empty.world
...
[Err] [Server.cc:530] Unregistered physics engine [ ], the default will be used instead

[jacobperron]

Reverting #1376 seems to resolve the issue.
I haven't looked too closely at what we can do to avoid using shell=True while avoiding these noisy console errors at the same time.

[jacobperron]

This was fixed in #1414