Simplify loading roles
Simplify loading roles

The same block of code was repeated several times in whole function.
Replace it with one, reusable function to prevent from possible
problems and simplify usage.

Change-Id: I4dbddcf1b361b59344a7bc15a793c568dba00f13

rdo-management/tuskar,rdo-management/tuskar,rdo-management/tuskar