Plugin should fail with a configuration error when specified with multiple event hubs and no storage connection

[robbavey]

When the storage_connection is not set in the azure event hub configuration, the InMemoryLeaseManager is used. This appears to be incompatible with using > 1 eventhub set via the advanced configuration, reporting the initial lease to the first eventhub as lost when subsequent leases to further event hubs are discovered. This leads to subscriptions to the first event hub being lost, and no further events from that eventhub will be processed.