Make HDFS-14617 (fsimage sub-sections) off by default

In HDFS-14617 I incorrectly stated that a fsimage with sub-sections listed in the image summary section could be loaded without the HDFS-14617 patch applied. However that was not correct. If the cluster was upgraded to a version containing this feature, then upon downgrade the image will fail to load.

I believe the simplest solution to this problem is to have this feature disabled by default. That way, after upgrading to a version with the feature, the operator can make a decision to enable the feature. If they later want to downgrade, then they must:

1. Disable the feature
2. Save the namespace, which will create an image without sub-sections
3. Perform the downgrade.

Even though the steps to downgrade are simple, we cannot expect people to be aware of this, and hence it is safest to disable the feature by default.

The only alternative is to create a new image layout version, but that seems excessive for this feature, as the core contents of the image have not changed and the sub-sections can be removed by disabling the feature and saving the namespace.