HOSTEDCP-1467: ARO HCP - Add ability to specify subnet at nodepool level

[bryan-cox]

What this PR does / why we need it:
This PR allows the subnet to be set on a NodePool basis rather than just at the Hosted Cluster level.

This PR also adds a flag to define the initial NodePool's subnet name when creating an Azure cluster, and it adds a flag when creating Azure NodePools to the HyperShift CLI.

Which issue(s) this PR fixes:
Fixes HOSTEDCP-1467

Checklist

• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.
• This change includes docs.
• This change includes unit tests.

[openshift-ci-robot]

@bryan-cox: This pull request references HOSTEDCP-1467 which is a valid jira issue.

In response to this:

What this PR does / why we need it:
This PR allows the subnet to be set on a NodePool basis rather than just at the Hosted Cluster level.

This PR also adds a flag to define the initial NodePool's subnet name when creating an Azure cluster, and it adds a flag when creating Azure NodePools to the HyperShift CLI.

Which issue(s) this PR fixes:
Fixes HOSTEDCP-1467

Checklist

• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.
• This change includes docs.
• This change includes unit tests.

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[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bryan-cox

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[bryan-cox]

/test images
/test unit
/test verify

[bryan-cox]

/test images
/test unit
/test verify

[bryan-cox]

/test images
/test unit
/test verify

[csrwng]

/lgtm

[bryan-cox]

/retest

[openshift-ci]

@bryan-cox: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-ibmcloud-iks	5eace16	link	false	/test e2e-ibmcloud-iks
ci/prow/e2e-ibmcloud-roks	5eace16	link	false	/test e2e-ibmcloud-roks

Full PR test history. Your PR dashboard.

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. I understand the commands that are listed here.