Add option to set / skip CloudBuildNumber

[MattKotsenas]

Fixes #1189

This PR adds the ability to set / skip the CloudBuildNumber in both the Cake task and the CLI.

Common

• Adds a new bool cloudBuildNumber to CloudCommand::SetBuildVariables
   

Cake.GitVersioning

• Adds a new CloudBuildNumber to GetVersioningCloudSettings with the default value of true
• Makes the default values of AllVariables and CommonVariables explicit (but no change)

nbgv CLI

• Adds a new command line switch --skip-cloud-build-number / -b which if specified sets cloudBuildNumber to false.

Tests

• There weren't any automated tests to cover this scenario, so added some basic tests

[MattKotsenas]

I felt bad not writing tests for this, but I'm not happy with the tests that I wrote:

• They're tied to individual cloud providers, which feels wrong
• They test by side effect, assuming that if all + common + cloudBuildNumber are all off, the output will always be empty

I'm happy to refactor CloudCommand to take an ICloudBuild for testing, but the only ways I can think of are to either make a public constructor or make an internal one and add InternalsVisibleTo. Let me know if you have any preference.

[AArnott]

Thanks for this. I left a comment on a suggested test change, but otherwise I'm happy to take this.

I also removed the = false; property initializers you added since I don't care for explicitly setting defaults.