✨ Allow setting "UseExistingCluster" via environment

[ichekrygin]

This PR is an attempt to address feature request: #222
Non-breaking feature: sparkles (:sparkles:)

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[DirectXMan12]

/kind feature

Can you please tag the PR title according to https://github.com/kubernetes-sigs/controller-runtime/blob/master/VERSIONING.md#pr-process ? Thanks!

[DirectXMan12]

We tend to try and favor using structs directly with sane defaults instead of constructors, when possible. Can you make this work with the existing setup? If you need to make a breaking change, now would be the time to do it ;-)

[ichekrygin]

@DirectXMan12 thank you for looking into this.

I think I share the sentiment of using structs directly with sane defaults. In this case, however, the struct Environment already has a field UseExistingCluster which I am attempting to set on "client/user" behalf, hence the "constructor" implementation.
The motivation is(was) not to "leak" environment variable value testing into the "client" code.

The alternative, "constructor-less" solution may look a bit ugly and convoluted (or maybe not):

in server.go

func UseExistingCluster() bool {
	return strings.ToLower(os.Getenv(envUseExistingCluster)) == "true"
}

in client code:

func TestMain(m *testing.M) {
	t := &envtest.Environment{
		CRDDirectoryPaths: []string{filepath.Join("..", "..", "..", "..", "..", "config", "crds")},
                UseExistingCluster: envtest.UseExistingCluster(),
	}
...

@DirectXMan12 would you be open to "contsturctor" like solion? If so - I will update the PR with...
I guess: "Non-breaking feature: sparkles (:sparkles:)" - is it?

[DirectXMan12]

So, the other option is changing UseExistingCluster bool to UseExistingCluster *bool, since most people probably don't want to set UseExistingCluster anyway, the awkwardness of builtin-pointers isn't super onerous. Sometimes I miss being able to write Option<bool>...

[ichekrygin]

I think I am coming to a realization that this is best handled in each individual project, rather than being built in. Closing.