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

Bug 1260239: https://bugs.launchpad.net/gluster-swift/+bug/1260239

Change-Id: I9e4866cd7ad08698f427846be566ab2364ad4850
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6516
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>