General: Add ability to change user value for templates

[iLLiCiTiT]

Brief description

Use get_openpype_username to receive value for user key in template data collection.

Description

This function is in openpype for a long time but was not fully used. It just gives ability to change username for templates instead of usign user name from machine by changing it in local settings.

Documentation

Added "user" key to available template keys.

Testing notes:

1. Change username in local settings UI
2. Add "{user}" to any template (e.g. publish)
3. Do something in a way that template is filled (e.g. run publish)
4. Check the create filepath if contains changed username from local settings