pass id to ServerGroupsManager in ServerGroup.delete()

ServerGroup object delete currently fails because the manager object
requires the server group id and is instead passed the server group
object itself (copy-paste error). This patch changes it to pass the
id.

Change-Id: Ic6a4a38157f52343fbb3e75a02d1f7c6bef28d99
Closes-Bug: #1400494
Merge "pass id to ServerGroupsManager in ServerGroup.delete()"