Maya: Do not pass set to maya commands (fixes support for older maya versions)

[BigRoy]

Implementation

This fixes support for Maya 2018

Older versions of Maya do not allow set type to be passed to Maya commands and will result in e.g. "RuntimeError: # Syntax error: unexpected end ( at position 4 while parsing"

E.g. error as reported on discord:

# Traceback (most recent call last):
#   File "C:\Users\22DOGS\OpenPype\openpype\tools\mayalookassigner\app.py", line 253, in on_process_selected
#     assign_look_by_version(nodes, version_id=version["_id"])
#   File "C:\Users\22DOGS\OpenPype\openpype\hosts\maya\api\lib.py", line 1600, in assign_look_by_version
#     shader_nodes = get_container_members(container_node)
#   File "C:\Users\22DOGS\OpenPype\openpype\hosts\maya\api\lib.py", line 1519, in get_container_members
#     for ref in cmds.ls(members, exactType="reference", objectsOnly=True):
# RuntimeError: # Syntax error: unexpected end ( at position 4 while parsing:
#     set([u'Props_lookMain_01_RN'])
#        ^
: set([u'Props_lookMain_01_RN'])

Brief description

This fixes loaded version updating, shader assignments and more areas that relied on get_container_members.

Testing notes:

1. start maya versions
2. ensure updating versions of loaded content continues to work, and lookdev shader assignments still work

[BigRoy]

Maya 2018 does have a visual bug on the context menu in the Manager / Scene Inventory. Unrelated to this specific PR but just wanted to mention it:

Edit: This visual bug is also present in Maya 2019. It works fine in Maya 2020 and 2022.

[maxpareschi]

Works now for both loading and material assignments, tested in maya 2018.7

[maxpareschi]

Seem to work now, tested in Maya2018.7.
Both asset loading and look assignment work!

[antirotor]

It's working for me too, thanks!