The function BuildHooksEnv in LUInstanceRemove, calls the function
BuildInstanceHookEnvByObject which needs to query the config to get
the secondary_nodes/disks of an instance. The function BuildHooksEnv
will be called before and after the Exec function. The second time
it will fail to query for the secondary_nodes/disks of the instance as
the instance will not be available at the config any more.

This patch changes BuildInstanceHookEnvByObject to allow it to override
the secondary_nodes/disks of an instance.

Signed-off-by: Ilias Tsitsimpis <iliastsi@grnet.gr>
Reviewed-by: Jose A. Lopes <jabolopes@google.com>