feat: rotate-certs fails faster if invalid ssh params

[jadarsie]

Reason for Change:

Addressing some aks-engine rotate-certs feedback

• document azure-env parameter
• fail faster if --ssh-host or --linux-ssh-private-key are invalid
• delete previous doc page
• more explicit stdout messages

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:

[jackfrancis]

Why are we deleting these docs?

[jadarsie]

It has duplicated/stale information, I pushed another one with my previous PR.

https://github.com/Azure/aks-engine/blob/master/docs/topics/rotate-certs.md

[codecov]

Codecov Report

Merging #4252 (e0161ed) into master (9924649) will decrease coverage by 0.01%.
The diff coverage is 59.45%.

@@            Coverage Diff             @@
##           master    #4252      +/-   ##
==========================================
- Coverage   72.08%   72.07%   -0.02%     
==========================================
  Files         141      141              
  Lines       21712    21719       +7     
==========================================
+ Hits        15652    15653       +1     
- Misses       5103     5111       +8     
+ Partials      957      955       -2     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (-0.60%)	⬇️
pkg/api/vlabs/validate.go	80.90% <90.47%> (+0.17%)	⬆️
pkg/api/addons.go	98.10% <100.00%> (ø)
pkg/api/convertertoapi.go	94.02% <100.00%> (-0.04%)	⬇️
pkg/api/defaults.go	93.45% <100.00%> (-0.04%)	⬇️

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, jadarsie

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