chore: validate VHD availability before upgrade/scale on Azure Stack Hub

[jadarsie]

Reason for Change:

Completing work started in #2342. This change blocks an upgrade operation if any required OS image is not available in the target Azure Stack Hub PIR (Platform Image Repository).

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #4062 (aee236e) into master (7fdbef0) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4062      +/-   ##
==========================================
- Coverage   73.25%   73.19%   -0.07%     
==========================================
  Files         135      135              
  Lines       20542    20558      +16     
==========================================
- Hits        15048    15047       -1     
- Misses       4521     4539      +18     
+ Partials      973      972       -1     

Impacted Files	Coverage Δ
cmd/deploy.go	60.79% <0.00%> (-0.34%)	⬇️
cmd/scale.go	12.56% <0.00%> (-0.29%)	⬇️
cmd/upgrade.go	37.75% <0.00%> (-1.61%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7fdbef0...aee236e. Read the comment docs.

[jackfrancis]

/lgtm