Allow external nodes to be added even when the replicaset is not exposed

Summary 

The PSMDB CR creation fails when the replicaset is not exposed, but externalNodes are specified.

Justification

Exposing replicasets in any particular MDB cluster should not dictate whether
external nodes can be added or not. The only requirement for the external node
should be that it is accessible on the network.

The operator should not fail validation if external nodes are specified, but the
replica set is not exposed.
 
Should have the same/similar implementation for

spec.replsets.[]

and

spec.sharding.configsvrReplSet

 Implementation Suggestion

Throw a warning during the CR validation instead of a hard failure.