feat: add human-readable output option to "get-versions" command

[mboersma]

Reason for Change:

The aks-engine get-versions command added in #448 defaults to JSON output that is hard to read. This adds the --output=human argument which acts similarly to az aks get-versions --output table:

$ ./bin/aks-engine get-versions -o human
Version        Upgrades
1.14.0-alpha.1 
1.13.3         1.14.0-alpha.1
1.13.2         1.13.3, 1.14.0-alpha.1
1.12.5         1.13.2, 1.13.3
1.12.4         1.12.5, 1.13.2, 1.13.3
1.11.7         1.12.4, 1.12.5
1.11.6         1.11.7, 1.12.4, 1.12.5
1.10.13        1.11.6, 1.11.7
1.10.12        1.10.13, 1.11.6, 1.11.7
1.9.11         1.10.12, 1.10.13
1.9.10         1.9.11, 1.10.12, 1.10.13
1.6.9          1.9.10, 1.9.11

Issue Fixed:

Fixes #520

Requirements:

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

Notes:
Should we make the table output the default?

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[codecov]

Codecov Report

Merging #527 into master will increase coverage by 0.07%.
The diff coverage is 84.09%.

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
+ Coverage   54.58%   54.66%   +0.07%     
==========================================
  Files          97       97              
  Lines       14641    14672      +31     
==========================================
+ Hits         7992     8020      +28     
- Misses       5974     5976       +2     
- Partials      675      676       +1

[jackfrancis]

lgtm

[mboersma]

/hold

I'm adding some tests.

Should -o table be the default?

[mboersma]

Team feedback was that -o table should be the default, so I'll make that change.

[jackfrancis]

lgtm

[mboersma]

I notice that aks-engine version has a similar argument, but it expects human or json as the value. I like things being predictable. Should we switch table to human, or vice versa?

[jackfrancis]

I think human is the more general term, for example it would be inclusive of output that wasn't tabular in nature.

[mboersma]

I changed "table" to "human" for parity with the existing aks-engine version command, and made the help messages parallel:

$ ./bin/aks-engine get-versions -h
Display supported Kubernetes versions and upgrade versions

Usage:
  aks-engine get-versions [flags]

Flags:
  -h, --help             help for get-versions
  -o, --output string    Output format. Allowed values: human, json (default "human")
      --version string   Kubernetes version (optional)
      --windows          Kubernetes cluster with Windows nodes (optional)

Global Flags:
      --debug   enable verbose debug logs
$ ./bin/aks-engine version -h
Print the version of AKS Engine

Usage:
  aks-engine version [flags]

Flags:
  -h, --help            help for version
  -o, --output string   Output format. Allowed values: human, json (default "human")

Global Flags:
      --debug   enable verbose debug logs

[mboersma]

I'm keeping this PR on hold just so I can merge it myself and ensure the first commit doesn't become the CHANGELOG entry.

[jackfrancis]

lgtm, ready for admin merge