Add InstanceTemplate field to GceInstance

[morshielt]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Allows access to Instance Template name in GCE Cloud Provider implementation.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

Hi @morshielt. Thanks for your PR.

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[BigDarkClown]

Just a single nit on the naming, otherwise looks good.

[BigDarkClown]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BigDarkClown, morshielt

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• cluster-autoscaler/cloudprovider/gce/OWNERS [BigDarkClown]

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