kubeadm: fix RequiredIPVSKernelModulesAvailable warning message

RequiredIPVSKernelModulesAvailable warning confuses users suggesting
that the IPVS proxier will not be used, which is not always the case.

Made the warning message less confusing:

        [WARNING RequiredIPVSKernelModulesAvailable]:
The IPVS proxier may not be used because the following required kernel
modules are not loaded: [ip_vs_rr ip_vs_wrr ip_vs_sh]
or no builtin kernel ipvs support was found: map[ip_vs_wrr:{}
ip_vs_sh:{} nf_conntrack_ipv4:{} ip_vs:{} ip_vs_rr:{}].
However, these modules may be loaded automatically by kube-proxy for you
if they are available on your system.

To verify IPVS support:

   Run "lsmod | grep 'ip_vs\|nf_conntrack'" and verify each of the above
modules are listed.

If they are not listed, you can use the following methods to load them:

1. For each missing module run 'modprobe $modulename' (e.g., 'modprobe
ip_vs', 'modprobe ip_vs_rr', ...)
2. If 'modprobe $modulename' returns an error, you will need to install
the missing module support for your kernel.

Fixes: kubernetes/kubeadm#975

pkg/util/ipvs/BUILD

@@ -41,6 +41,7 @@ go_library(
         "@io_bazel_rules_go//go/platform:linux": [
             "//staging/src/k8s.io/apimachinery/pkg/util/sets:go_default_library",
             "//vendor/github.com/docker/libnetwork/ipvs:go_default_library",
+            "//vendor/github.com/lithammer/dedent:go_default_library",
             "//vendor/k8s.io/klog:go_default_library",
         ],
         "//conditions:default": [],

pkg/util/ipvs/kernelcheck_linux.go

@@ -26,6 +26,7 @@ import (
 	"k8s.io/apimachinery/pkg/util/sets"
 	utilsexec "k8s.io/utils/exec"
 
+	"github.com/lithammer/dedent"
 	"k8s.io/klog"
 )
 
@@ -80,10 +81,20 @@ func (r RequiredIPVSKernelModulesAvailableCheck) Check() (warnings, errors []err
 			}
 		}
 		if len(builtInModules) != 0 {
-			warnings = append(warnings, fmt.Errorf(
+			warnings = append(warnings, fmt.Errorf(dedent.Dedent(`
-				"the IPVS proxier will not be used, because the following required kernel modules are not loaded: %v or no builtin kernel ipvs support: %v\n"+
+
-					"you can solve this problem with following methods:\n 1. Run 'modprobe -- ' to load missing kernel modules;\n"+
+				The IPVS proxier may not be used because the following required kernel modules are not loaded: %v
-					"2. Provide the missing builtin kernel ipvs support\n", modules, builtInModules))
+				or no builtin kernel IPVS support was found: %v.
+				However, these modules may be loaded automatically by kube-proxy if they are available on your system.
+				To verify IPVS support:
+
+				   Run "lsmod | grep 'ip_vs|nf_conntrack'" and verify each of the above modules are listed.
+
+				If they are not listed, you can use the following methods to load them:
+
+				1. For each missing module run 'modprobe $modulename' (e.g., 'modprobe ip_vs', 'modprobe ip_vs_rr', ...)
+				2. If 'modprobe $modulename' returns an error, you will need to install the missing module support for your kernel.
+				`), modules, builtInModules))
 		}
 	}