Add GetKernelVersion to ipvs.KernelHandler interface

ipvs `getProxyMode` test fails on mac as `utilipvs.GetRequiredIPVSMods`
try to reach `/proc/sys/kernel/osrelease` to find version of the running
linux kernel. Linux kernel version is used to determine the list of required
kernel modules for ipvs.

Logic to determine kernel version is moved to GetKernelVersion
method in LinuxKernelHandler which implements ipvs.KernelHandler.
Mock KernelHandler is used in the test cases.

Read and parse file is converted to go function instead of execing cut.

cmd/kube-proxy/app/server_test.go

@@ -73,13 +73,18 @@ func (fake *fakeKernelCompatTester) IsCompatible() error {
 
 // fakeKernelHandler implements KernelHandler.
 type fakeKernelHandler struct {
-	modules []string
+	modules       []string
+	kernelVersion string
 }
 
 func (fake *fakeKernelHandler) GetModules() ([]string, error) {
 	return fake.modules, nil
 }
 
+func (fake *fakeKernelHandler) GetKernelVersion() (string, error) {
+	return fake.kernelVersion, nil
+}
+
 // This test verifies that NewProxyServer does not crash when CleanupAndExit is true.
 func TestProxyServerWithCleanupAndExit(t *testing.T) {
 	// Each bind address below is a separate test case