Upgrade the CLI to use Go 1.12.

[armandgrillet]

This does not change our supported operating systems, Go 1.12 being the
last release supporting macOS 10.10 Yosemite.

[armandgrillet]

@mesosphere-ci run integration tests

[armandgrillet]

Waiting for docker-library/golang#264 to be merged.

[bamarni]

The new ProcessState.ExitCode method returns the process's exit code.

We should be able to simplify this by using exitErr.ExitCode():

dcos-cli/pkg/cmd/dcos.go

Lines 234 to 244 in b4a7d91

	// When the plugin command exits with a non-zero code, the main CLI process
	// exit code should be the same.
	//
	// see https://jira.mesosphere.com/browse/DCOS_OSS-4399
	if exiterr, ok := err.(*exec.ExitError); ok {
	if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
	// TODO: have a more generic error handling system
	// where errors can be associated with an exit code.
	os.Exit(status.ExitStatus())
	}
	}

Wanna give it a try in this PR?

[armandgrillet]

@mesosphere-ci run integration tests

[armandgrillet]

@mesosphere-ci run integration tests

[bamarni]

LGTM, we will have to wait for Appveyor to support Go 1.12. If they do not do it soon we'll investigate on alternative.

[armandgrillet]

For Appveyor: appveyor/ci#2875

[armandgrillet]

@mesosphere-ci run integration tests

[armandgrillet]

@mesosphere-ci run integration tests