Fix bug where admin is able to delete reseller_admin

Changed the code to only allow the super_admin to delete a reseller_admin.
This follows the same logic of user creation, where only the super_admin
can create a reseller_admin.

Also, took the opportunity to refactor some code and implemented
get_user_detail method to remove duplicated code

Signed-off-by: Thiago da Silva <thiago@redhat.com>