During model thumbnail creation, it was possible to encounter an
endless loop of the message:

"Couldn't find an AF_INET address for []"

which was caused when there were plugins in the SDF files that may
require ROS to be initialized to load properly. This patch creates a
copy of the model SDF and removes all plugins during thumbnail creation,
which eliminates the issue and still produces the same thumbnail.

This patch also cleans up the formatting up the thumbnail script and
fixes a few other minor errors that may result in console warnings/errors.

Change-Id: I90e8c43035f4d15a25cb3da27042ddc58614cd5e