Walter 1.2.0 Linux Centos7 build

New features

• Build system: Added option in vfx_platform_builder script to disable
  requirement for Houdini or Katana plugins.
• Katana: Handle USD purposes in KatanaPlugin.
  "guide" and "proxy" are viewed in the viewport but not in renderer.
  "render" is viewed everywhere (viewport and render).
• Katana: Handle USD Lights.
  All lights are scanned and put into a lightList at /root/world in Katana
  to ensure the renderer find them. Since UsdLuxLight is not a concrete schema,
  we made an ArnoldLight that inherits from UsdLuxLight and allow us to have generic
  ArnoldLight into USD scene.
• Maya: Variants support in the Walter outliner.
• Arnold: primvar support for Usd PointInstancer

Bug fixes

• Arnold: Texture coords stretching issue resolved.
• Katana: crash when scene graph is evaluated and layers are changed.
• Build system: The 'debug' flag is now propagated to every dependencies

Removed

• Arnold: Version 4 is not supported anymore.
• MtoA Extension: jsoncpp dependency not needed anymore.

Build for:

• Arnold-5.1.1.1
• Houdini-16.5.350
• HtoA-3.0.3
• Katana-3.0.v1
• KtoA-2.1.0.0
• Maya-2018.3
• MtoA-3.0.1.1