Refactor metadata handling logic

[leakingtapan]

We need to refactor how EC2 metadata is passed around in our initialization logic.

Per conversation from e2e test PR:

The problem with that is that its going to leak AWS implementation details into this driver code.

Another problem is that `EC2Metadata` returned by `svc := ec2metadata.New(sess)` is also required in `&ec2rolecreds.EC2RoleProvider{Client: svc},`, passing in both also doesn't seem right. 

I feel the way we are handling metadata is a bit messy. Will need to refactor later to improve this.

Originally posted by @dkoshkin in #173

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[leakingtapan]

/remove-lifecycle stale

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale

[fejta-bot]

Stale issues rot after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle rotten.
Rotten issues close after an additional 30d of inactivity.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle rotten

[leakingtapan]

/close

[k8s-ci-robot]

@leakingtapan: Closing this issue.

In response to this:

/close

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.