fix: always use /var/run/reboot-required

[jackfrancis]

Reason for Change:

This PR standardizes the CSE bootstrapping script to always evaluate [ -f /var/run/reboot-required ] (does the file exist?) to determine whether or not we need to reboot.

In addition, in the case where we want to reboot, we touch that file, which as far as I can tell is a safe tactic as /var/run is tmpfs and files therein will be reliably removed upon reboot. Cf:

azureuser@k8s-master-31453872-1:~$ ls -la /var/run
lrwxrwxrwx 1 root root 4 Nov 13  2019 /var/run -> /run
azureuser@k8s-master-31453872-1:~$ df -h /run
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           796M  1.2M  795M   1% /run

Issue Fixed:

Fixes #3533

Requirements:

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

Notes:

[jackfrancis]

cc @MichaelSinz

[codecov]

Codecov Report

Merging #3536 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3536   +/-   ##
=======================================
  Coverage   73.19%   73.19%           
=======================================
  Files         147      147           
  Lines       25079    25079           
=======================================
  Hits        18356    18356           
  Misses       5589     5589           
  Partials     1134     1134           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	54.08% <ø> (ø)

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[Michael-Sinz]

Looks good to me.

[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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