Maya: Creating Arnold shading nodes by script

When creating Arnold shading nodes by script, use shadingNode command instead of createNode command. (Maya2018)

# shadingNode command
cmds.shadingNode('aiStandardSurface',asShader=True,n="arnoldNode1");

# createNode command
# This node will not be visible in Hypershade window.
cmds.createNode('aiStandardSurface',n="arnoldNode2");