Fix instance metadata + vmss default behavior for k8s 1.10.0 to 1.10.2

[CecileRobertMichon]

What this PR does / why we need it: Instance metadata + VMSS + k8s version < 1.10.2 doesn't work. So far there was validation to ensure that a user explicitly specifies AvailabilitySet or instance meta data = false with k8s 1.10.0 and 1.10.1. This PR makes it easier by assigning the default according to those rules. New default behavior is:

• k8s version < 1.10.0: AvailabilitySet, useInstanceMetadata: true

• k8s version == 1.10.0 or 1.10.1: AvailabilitySet, useInstanceMetadata: true, unless useInstanceMetadata is set to false, then VMSS is default

• k8s version >= 1.10.2: VMSS, useInstanceMetadata: true

To summarize, useInstanceMetadata is always set to true by default if the value is empty. The availabilityProfile depends on whether VMSS is supported with the other options.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

If applicable:

• documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

[jackfrancis]

/lgtm

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This pull-request has been approved by: CecileRobertMichon, jackfrancis

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