(MODULES-2637) Checks java actually installed

The other method for running which java doesn't work on OSX, as java is installed as an empty shim when not installed in the base OSX image:

```
vagrant:~ vagrant$ java
No Java runtime present, requesting install.
vagrant:~ vagrant$ /usr/libexec/java_home --failfast
Unable to find any JVMs matching version "(null)".
vagrant:~ vagrant$
```

We can use the `/usr/libexec/java_home --failfast` command to check if
java is actually present first.

Originally-by: Peter Souter <peter.souter@puppetlabs.com>

lib/facter/java_version.rb

@@ -12,12 +12,23 @@
 # Notes:
 #   None
 Facter.add(:java_version) do
+  # the OS-specific overrides need to be able to return nil,
+  # to indicate "no java available". Usually returning nil
+  # would mean that facter falls back to a lower priority
+  # resolution, which would then trigger MODULES-2637. To
+  # avoid that, we confine the "default" here to not run
+  # on those OS.
+  # Additionally, facter versions prior to 2.0.1 only support
+  # positive matches, so this needs to be done manually in setcode.
   setcode do
-    if Facter::Util::Resolution.which('java')
+    unless [ 'openbsd', 'darwin' ].include? Facter.value(:operatingsystem).downcase
-      Facter::Util::Resolution.exec('java -Xmx8m -version 2>&1').lines.first.split(/"/)[1].strip
+      if Facter::Util::Resolution.which('java')
+        Facter::Util::Resolution.exec('java -Xmx8m -version 2>&1').lines.first.split(/"/)[1].strip
+      end
     end
   end
 end
+
 Facter.add(:java_version) do
   confine :operatingsystem => 'OpenBSD'
   has_weight 100
@@ -29,3 +40,13 @@ Facter.add(:java_version) do
     end
   end
 end
+
+Facter.add(:java_version) do
+  confine :operatingsystem => 'Darwin'
+  has_weight 100
+  setcode do
+    unless /Unable to find any JVMs matching version/ =~ Facter::Util::Resolution.exec('/usr/libexec/java_home --failfast 2>&1')
+      Facter::Util::Resolution.exec('java -Xmx8m -version 2>&1').lines.first.split(/"/)[1].strip
+    end
+  end
+end

spec/unit/facter/java_version_spec.rb

@@ -23,6 +23,22 @@ OpenJDK 64-Bit Server VM (build 24.71-b01, mixed mode)
           expect(Facter.value(:java_version)).to eq("1.7.0_71")
         end
       end
+      context 'on Darwin' do
+        before do
+          Facter.fact(:operatingsystem).stubs(:value).returns("Darwin")
+        end
+        let(:facts) { {:operatingsystem => 'Darwin'} }
+        it do
+          java_version_output = <<-EOS
+java version "1.7.0_71"
+Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
+Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)
+          EOS
+          Facter::Util::Resolution.expects(:exec).with("/usr/libexec/java_home --failfast 2>&1").returns("/Library/Java/JavaVirtualMachines/jdk1.7.0_71.jdk/Contents/Home")
+          Facter::Util::Resolution.expects(:exec).with("java -Xmx8m -version 2>&1").returns(java_version_output)
+          Facter.value(:java_version).should == "1.7.0_71"
+        end
+      end
       context 'on other systems' do
         before do
           Facter.fact(:operatingsystem).stubs(:value).returns("MyOS")
@@ -51,6 +67,16 @@ Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)
           expect(Facter.value(:java_version)).to be_nil
         end
       end
+      context 'on Darwin' do
+        before do
+          Facter.fact(:operatingsystem).stubs(:value).returns("Darwin")
+        end
+        let(:facts) { {:operatingsystem => 'Darwin'} }
+        it do
+          Facter::Util::Resolution.expects(:exec).at_least(1).with("/usr/libexec/java_home --failfast 2>&1").returns('Unable to find any JVMs matching version "(null)".')
+          Facter.value(:java_version).should be_nil
+        end
+      end
       context 'on other systems' do
         before do
           Facter.fact(:operatingsystem).stubs(:value).returns("MyOS")