Unclear if gateway-info is a hard requirement

[DnPlas]

_get_env_vars() calls _get_istio_gateway() which returns the name and namespace of the istio ingress gateway as a string in the namespace/name format. The code suggests that in the case of the latter returning None, _get_env_vars() will not stop its execution, but in _get_istio_gateway() it is suggested that the unit should go to WaitingStatus if the relation with istio-pilot:gateway-info does not exist.

From the code, it is not clear if the gateway-info is a hard requirement that should put the unit into WaitingStatus or even BlockedStatus, the only thing that is clear is that _get_env_vars() won't care if there is information about the gateway and it will just proceed. In the case of the relation not being necessary 100% of the times, I recommend we reflect that in the code (removing the WaitingStatus) and also in the documentation.