oci: respect process spec on exec

This patch fixes exec to use the original (start-time) process exec
configuration. Otherwise, we were creating a brand new spec process w/o
additional groups for instance.
Spotted while integrating CRI-O with cri-test...The test was failing
with:
```
• Failure [10.640 seconds]
[k8s.io] Security Context
/home/amurdaca/go/src/github.com/kubernetes-incubator/cri-tools/pkg/framework/framework.go:72
  bucket
  /home/amurdaca/go/src/github.com/kubernetes-incubator/cri-tools/pkg/validate/security_context.go:407
    runtime should support SupplementalGroups [It]
    /home/amurdaca/go/src/github.com/kubernetes-incubator/cri-tools/pkg/validate/security_context.go:272

    Expected
        <[]string | len:1, cap:1>: ["0"]
    to contain element matching
        <string>: 1234
```

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

server/container_create.go

@@ -1042,6 +1042,7 @@ func (s *Server) createSandboxContainer(ctx context.Context, containerID string,
 	if err != nil {
 		return nil, err
 	}
+	container.SetSpec(specgen.Spec())
 	container.SetMountPoint(mountPoint)
 
 	for _, cv := range containerVolumes {