build: make build with go 1.14

[sozercan]

Reason for Change:

Tried the steps in #2824 (comment) but looks like make build is still broken with go 1.14

Had the following error when executing go generate -mod=vendor -v ./...

go: inconsistent vendoring in /home/sozercan/go/src/github.com/Azure/aks-engine:
        golang.org/x/[email protected]: is explicitly required in go.mod, but vendor/modules.txt indicates golang.or
g/x/[email protected]
        golang.org/x/[email protected]: is marked as explicit in vendor/modules.txt, but not explicitly required in
go.mod

run 'go mod vendor' to sync, or use -mod=mod or -mod=readonly to ignore the vendor directory

Ran make vendor and make build successfully (also tried with go 1.13)

Issue Fixed:

#2812

Requirements:

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

Notes:

[jackfrancis]

/lgtm

[codecov]

Codecov Report

Merging #3005 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3005   +/-   ##
=======================================
  Coverage   72.12%   72.12%           
=======================================
  Files         141      141           
  Lines       26257    26257           
=======================================
  Hits        18937    18937           
  Misses       6227     6227           
  Partials     1093     1093           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	31.57% <0.00%> (ø)

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