Re-add pid in runtime config

This is so we can support downgrades, eg run old k0s binary against
newer k0s runtime config.

Probably meaningless because we don't really support downgrading to
older version(?)

Signed-off-by: Natanael Copa <ncopa@mirantis.com>

pkg/config/runtime.go

@@ -56,6 +56,7 @@ type RuntimeConfig struct {
 type RuntimeConfigSpec struct {
 	NodeConfig *v1beta1.ClusterConfig `json:"nodeConfig"`
 	K0sVars    *CfgVars               `json:"k0sVars"`
+	Pid        int                    `json:"pid"` // unused. Delete
 	lockFile   *os.File
 }
 
@@ -136,6 +137,7 @@ func NewRuntimeConfig(k0sVars *CfgVars) (*RuntimeConfig, error) {
 		Spec: &RuntimeConfigSpec{
 			NodeConfig: nodeConfig,
 			K0sVars:    k0sVars,
+			Pid:        os.Getpid(),
 			lockFile:   lockFile,
 		},
 	}