Make azure credentials optional in manager deployment

[CecileRobertMichon]

What type of PR is this?

What this PR does / why we need it: Following up on #1386 and #977, this makes the Azure credentials environment variables optional in the infrastructure components yaml spec by defaulting the values to "". When not setting the environment variables in the manager, the user will need to specify an AzureClusterIdentity for each workload cluster.

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 #

Special notes for your reviewer:

• this should be merged after Multitenancy for managed clusters #1386
  /hold
• this requires envsubst to be used to install CAPZ components (which is already required due to EXP feature flags)
• how can we make the UX better for new users who don't set manager variables and thus have to create an AzureClusterIdentity on their cluster ?

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

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Make azure credentials optional in manager deployment

[CecileRobertMichon]

/assign @nader-ziada @devigned

[devigned]

/lgtm

Yay! One step closer to no creds in the controller.

[nader-ziada]

should we wait until the issues with private cluster/dns thing is resolved?

[CecileRobertMichon]

should we wait until the issues with private cluster/dns thing is resolved?

I don't think that's necessarily a blocker since it can still be used with creds, although it would be nice to figure out that one before the release. Is there a tracking issue for the dns issue?

[CecileRobertMichon]

/hold cancel
/assign @nader-ziada

[nader-ziada]

I was hoping for #1360 to get merged in first, but I guess the order doesn't really matter

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nader-ziada

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[nader-ziada]

/test pull-cluster-api-provider-azure-e2e