Namespace SDK enums so that theirs strings are easier to read

[lpabon]

Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE

What happened:
SDK proto enums should be inside a message so that the strings are easier to comprehend. Instead of using:

openstorage/api/api.proto

Lines 2721 to 2724 in a0292eb

	enum SdkCloudBackupStatusType {
	// Unkonwn
	SdkCloudBackupStatusTypeUnknown = 0;
	// Not started

Use:

openstorage/api/api.proto

Lines 3129 to 3137 in a0292eb

	message CloudMigrate {
	enum OperationType {
	InvalidType = 0;
	// Migrate all volumes in the cluster
	MigrateCluster = 1;
	// Migrate a single volume
	MigrateVolume = 2;
	// Migrate a group of volumes
	MigrateVolumeGroup = 3;

See: #957 (comment)