instance-vsphere should support additional properties

[stealthybox]

@thebsdbox

Datastore, Cluster, and ResourcePool should be selectable for instance placement.
It seems not all environments have configured defaults for these fields.

As an example, my group is similar to #618 and fails to deploy, because multiple datacenters are available:

INFO[4147] Adding 1 instances to group to reach desired 1
ERRO[0519] Error:
No Datacenter instance could be found inside of vCenter default datacenter resolves to multiple instances, please specify
ERRO[4147] Failed to provision: No Datacenter instance could be found inside of vCenter default datacenter resolves to multiple instances, please specify
INFO[4150] Ignored group 'Infrakit_Deployment'
WARN[0522] No Instances found

Asking the user to specify these values is probably the simplest solution.

@bdeluca also had some interesting requests around round-robining across enums when multiple are available which could potentially be a separate feature.

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Plugin: instance-vsphere / flavor-vanilla / group-default
Infrakit Commit: c8080c9
vSphere Version: 6.5

[thebsdbox]

Added the capabilities in #624

There has been additional capabilities added since, which the scripting engine will consume.