feat: Add support for custom runc deb URL

[cpuguy83]

Reason for Change:

This helps enable testing new runc packages on aks-engine clusters.

Issue Fixed:

Credit Where Due:

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

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #4403 (7bd80b7) into master (60828c7) will increase coverage by 0.10%.
The diff coverage is 96.93%.

@@            Coverage Diff             @@
##           master    #4403      +/-   ##
==========================================
+ Coverage   72.04%   72.14%   +0.10%     
==========================================
  Files         141      141              
  Lines       21631    21722      +91     
==========================================
+ Hits        15584    15672      +88     
- Misses       5096     5099       +3     
  Partials      951      951              

Impacted Files	Coverage Δ
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/api/types.go	92.85% <ø> (ø)
pkg/api/vlabs/types.go	73.04% <ø> (ø)
pkg/api/vlabs/validate.go	81.83% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
pkg/engine/template_generator.go	68.37% <50.00%> (-0.17%)	⬇️
pkg/api/addons.go	98.01% <100.00%> (ø)
pkg/api/converterfromapi.go	95.71% <100.00%> (+<0.01%)	⬆️
pkg/api/convertertoapi.go	94.04% <100.00%> (+0.01%)	⬆️
pkg/api/k8s_versions.go	100.00% <100.00%> (ø)

---

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpuguy83, jackfrancis

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