The ESX5 driver doesn't have a way of mapping the network name to its device name because a .vmx template uses different field names for it and so packer let's ESX handle filling this in. This patch will check to see if the driver that packer determines is missing a NetworkMapper implementation (by checking for nil). If it is, then fall back to using "nat" despite ESX not using the network type at all. This is what packer did prior to exposing the network type to the user back in version 1.1.3.
This closes issue #5916.
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