fix: idempotent systemd + disk mount for etcd

[jackfrancis]

Reason for Change:

This PR includes "retry etcd disk volume mount" when starting etcd systemd service as the systemd service launch depends upon the disk mount.

Issue Fixed:

Fixes #3125

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3126 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3126   +/-   ##
=======================================
  Coverage   71.04%   71.05%           
=======================================
  Files         147      147           
  Lines       25679    25687    +8     
=======================================
+ Hits        18244    18252    +8     
  Misses       6297     6297           
  Partials     1138     1138           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	37.76% <ø> (ø)
pkg/api/vlabs/validate.go	79.27% <0.00%> (ø)
pkg/api/defaults.go	92.48% <0.00%> (+0.01%)	⬆️
pkg/api/addons.go	97.85% <0.00%> (+0.01%)	⬆️

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[mboersma]

/lgtm