Title: in the case where a VHDS config is sent by management server after the initManager has been initialized, the RDS config update end in programming error.

Description:

At present a VHDS subscription is started by initManager.initialize(), in the case a newly added VHDS is sent after the initManager been initialized, it ends in programming error when trying to add the init_target_ to the init manager.

Repro steps:

1. Management server sends RouteConfiguration w/o VHDS
2. initManager initialize() called, Envoy starts serving.
3. Management server sends RouteConfiguration w/ VHDS
4. RDS impl tries to create a VHDS subscription, adds an init-target to the initManager which has been initialized, causes an assert failure.