Fix panic when datacenter obj is not found

[rahulbabu95]

Description of changes:
When the config parser is unable to find a object such as Datacenter/MachineConfig due to a typo in the DatacenterRef name (or if it was never passed in the spec), parser just doesn't set that object and doesn't do any validations around it. Accessing those objects later leads to a panic at runtime.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x148 pc=0x103493f20]

This change adds validations on the config, around the Datacenter obj if it's not found in the config to explicitly error out.

Error: unable to get cluster config from file: invalid cluster config: VSphereDatacenterConfig name rahul-130-br-tst1 not found

Fixes #7531

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.50%. Comparing base (31c6854) to head (c07f0f5).

Files	Patch %	Lines
pkg/cluster/cloudstack.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8494   +/-   ##
=======================================
  Coverage   73.49%   73.50%           
=======================================
  Files         578      578           
  Lines       36504    36514   +10     
=======================================
+ Hits        26829    26840   +11     
+ Misses       7956     7954    -2     
- Partials     1719     1720    +1     

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[rahulbabu95]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rahulbabu95

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[rahulbabu95]

/cherrypick release-0.20

[rahulbabu95]

/cherrypick release-0.19

[eks-distro-pr-bot]

@rahulbabu95: new pull request created: #8495

In response to this:

/cherrypick release-0.20

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[eks-distro-pr-bot]

@rahulbabu95: new pull request created: #8496

In response to this:

/cherrypick release-0.19

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.