Don't attempt to load GafferRenderMan or GafferArnold unless available

[bentoogood]

we should probably attempt to detect the relevant env vars that would indicate arnold/renderman availability, and skip these imports if there's no sign.

ERROR : startup/gui/menus.py : Error loading Arnold module - "libai.so: cannot open shared object file: No such file or directory".
 Traceback (most recent call last):
  File "/gaffer-0.72.2-posix/startup/gui/menus.py", line 68, in <module>
    import GafferArnold
  File "/gaffer-0.72.2-posix/python/GafferArnold/__init__.py", line 48, in <module>
    from _GafferArnold import *
ImportError: libai.so: cannot open shared object file: No such file or directory

ERROR : startup/gui/menus.py : Error loading RenderMan module - "lib3delight.so: cannot open shared object file: No such file or directory".
 Traceback (most recent call last):
  File "/gaffer-0.72.2-posix/startup/gui/menus.py", line 93, in <module>
    import GafferRenderMan
  File "/gaffer-0.72.2-posix/python/GafferRenderMan/__init__.py", line 55, in <module>
    from _GafferRenderMan import *
ImportError: lib3delight.so: cannot open shared object file: No such file or directory