linux: Make containerd less runc specific

We hope that containerd supports any OCI compliant runtime, and not only
runc.
This patch fixes all the error messages to not be completely runc
specific and change the initProcess structure to have its runtime
pointer be called 'runtime' and not 'runc'

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

linux/shim/service.go

@@ -383,7 +383,7 @@ func (s *Service) waitExit(p process, pid int, cmd *reaper.Cmd) {
 }
 
 func (s *Service) getContainerPids(ctx context.Context, id string) ([]uint32, error) {
-	p, err := s.initProcess.runc.Ps(ctx, id)
+	p, err := s.initProcess.runtime.Ps(ctx, id)
 	if err != nil {
 		return nil, err
 	}