move account_user role gathering into method
move account_user role gathering into method

this is part 2 of a series of commits to identify the proper roles on
the proper shard. This has no logic changes and existing specs should
pass. 

test plan
 - specs should pass

flag=none
refs VICE-2128

Change-Id: I8d125140e79959a8fde26b0b7af4d2b5381374f5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/278702
Reviewed-by: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>
QA-Review: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>
Product-Review: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms