GCE: Implement kube-env caching

[BigDarkClown]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Caching of kube-env for instance templates. The change greatly improves the CPU usage for idle clusters, as ~80% of the main loop time is spend on repetitive unmarshalling of kube-env.

The improvement is drastic for idle clusters even with only 3 MIGs. In my testing the loop time decreased from 1.25s to 0.25s.

[atwamahmoud]

In General, LGTM
However I might be messing some stuff so it'd be safer if someone else took a quick look

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[atwamahmoud]

/lgtm

It would be better to omit the Get from function names but it's out of scope for this PR since we'll have to rename not just added functions but older ones to remain consistent

Edit: Ahh, I forgot I'm not in the OWNERS file passing to @jayantjain93

[k8s-ci-robot]

@atwamahmoud: changing LGTM is restricted to collaborators

In response to this:

/lgtm

It would be better to omit the Get from function names but it's out of scope for this PR since we'll have to rename not just added functions but older ones to remain consistent

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[x13n]

Will we ever need either Invalidate ? Since instance templates are immutable, we probably won't ever call this. What we might want instead though is some expiration mechanism - to avoid memleaks in clusters with high MIG churn. This is a nice to have though, I don't expect single CA instance to observe enough instance templates over entire process lifetime to visibly inflate memory usage.

[x13n]

I left a comment, but that is something to follow up on later, not in this PR, so let's merge as is.

/lgtm

Btw, @atwamahmoud - you don't need to be an OWNER to /lgtm, you just need to be a Kubernetes org member.