Refactor resource manager to use interface and override resource manager endpoint URL

[Karthik-K-N]

What this PR does / why we need it:

This PR refactors to user interface for resource manager related operations, This will allow help us while implementing unit tests.
Also allows users to override the resource manager endpoint URL.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes ##1731

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Allow users to override the resource manager endpoint URL

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[mkumatag]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Karthik-K-N, mkumatag

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