WIP - openstack: try to fetch Metadata service first

[EmilienM]

In most cases, OpenStack metadata service is available in the cloud,
which removes the requirement of config-drive.

This patch will change the logic to try reading Metadata and Network data by
safely fetching the URL first and if it's not available, then it'll try
to read from the file provided by config-drive.

Note: it's adding a new dependency on hashicorp/go-retryablehttp which
is a library which provides a HTTP client interface with automatic retries
and exponential backoff.

[openshift-ci]

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[openshift-ci]

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