plugins: misc fixes

Rename variable to reflect manifest -> config renaming
Populate Description fields when computing privileges.
Refactor/reuse code from daemon/oci_linux.go

Signed-off-by: Tibor Vass <tibor@docker.com>

oci/namespaces.go

@@ -0,0 +1,16 @@
+package oci
+
+import specs "github.com/opencontainers/runtime-spec/specs-go"
+
+// RemoveNamespace removes the `nsType` namespace from OCI spec `s`
+func RemoveNamespace(s *specs.Spec, nsType specs.NamespaceType) {
+	idx := -1
+	for i, n := range s.Linux.Namespaces {
+		if n.Type == nsType {
+			idx = i
+		}
+	}
+	if idx >= 0 {
+		s.Linux.Namespaces = append(s.Linux.Namespaces[:idx], s.Linux.Namespaces[idx+1:]...)
+	}
+}