Asset path redirection updates

[annatisch]

• Better renderer-specific control on both asset pre-render scripting and job settings
• Fixed bug in thumbnail script where if the render fails and there's no outputs, no "thumbs" directory gets created which causes an output file upload error.
• Better Vray file redirection by adding env variable search path list. Also added back use of the pre-render MEL as the errors in this seem to be resolved in the latest releases.
• Better Arnold file redirection by stripping full path from aiImage node paths in pre-render MEL.