[enhancement] Update Java driver fingerprinting for macOS

[angrycub]

On versions of macOS without Java installed, there is a stub executable that presents a warning to users that there is no longer a default JDK installed on macOS.

The fingerprinter runs this stub command every 30 seconds while Nomad is running because of

nomad/drivers/java/driver.go

Lines 29 to 30 in 65f09ed

	// fingerprintPeriod is the interval at which the driver will send fingerprint responses
	fingerprintPeriod = 30 * time.Second

nomad/drivers/java/utils.go

Lines 11 to 27 in 65f09ed

	var javaVersionCommand = []string{"java", "-version"}
	func javaVersionInfo() (version, runtime, vm string, err error) {
	var out bytes.Buffer
	cmd := exec.Command(javaVersionCommand[0], javaVersionCommand[1:]...)
	cmd.Stdout = &out
	cmd.Stderr = &out
	err = cmd.Run()
	if err != nil {
	err = fmt.Errorf("failed to check java version: %v", err)
	return
	}
	version, runtime, vm = parseJavaVersionOutput(out.String())
	return
	}

It appears that we could fingerprint for an installed JDK (that would be seen by this stub) by looking for one or more symlinks in /Library/Java/JavaVirtualMachines. This would definitely improve the getting started experience on macOS.

[vancluever]

@angrycub and I were discussing workarounds here, so I thought I'd post one here that I just applied: you can remove the /usr/bin/java symlink to make this go away. Not too sure if it comes back eventually though (ie: through patches or with the next version of MacOS).

On modern MacOS you need to disable SIP from recovery mode and remount / read-write (mount -u -w /) before you can operate on the symlink. Of course, you can just delete it from recovery mode too.

[angrycub]

I was able to collect enough of the necessary bits to make a null.jdk that can be put in /Library/Java/JavaVirtualMachines that uses symlinks to /usr/bin/false as the target command.
Tested on macOS Catalina 10.15.4.

Unzip and then sudo cp -R null.jdk /Library/Java/JavaVirtualMachines

null.jdk.zip

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