API key via kubernetes secret

[mkumatag]

What this PR does / why we need it:

This PR will help the user to supply the API key via k8s secret and the controller will use the API key via that secret to talk to the IBM Cloud.

Typical flow:

$ kustomize build config/default > infrastructure-components.yaml
# manually edit the file and replace the `${IBMCLOUD_API_KEY}` with a valid key
# this will be replaced on the fly when we use clusterctl init command in the future.
$ kubectl apply -f infrastructure-components.yaml

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: #366

Special notes for your reviewer:

/area provider/ibmcloud

Release note:

[mkumatag]

/cc @wentao-zh

[wentao-zh]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mkumatag, wentao-zh

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