Finding by specify server group name failed in ServerGroupsManager.find(),
ServerGroup.NAME_ATTR is "server_group_name" currently, but NAME_ATTR
should be "name", because only "name" attribute exists in ServerGroup
object, not "server_group_name".
Change-Id: Id6b7676d14e6283d856a069da5ff287dc3228386
Closes-Bug: #1563301