move datacenter out of auth vars for manage_folder

[mikemorency]

This change is from the testathon feedback.

It includes two issues:

1. The documentation for the manage_folder role had the datacenter variable in the wrong section
2. I noticed the datacenter for this role is inconsistent with other roles. So i fixed that too

I updated the docs and changed the variable to match other roles, while keeping the old variable name valid

[anna-savina]

LGTM