More precautions should be taken around updating of legacy apiEndpoint field

[cywang117]

Precautions involve the following:

• Enable persistent logging (this should be documented as a suggested user-run recommendation)
• Back up volume data on the device prior to running configizer (also documented as a suggested user-run recommendation)
• Stop the Supervisor service prior to modification

We have observed a device losing its volumes & images from migrating resin.io -> balena-cloud.com, without any logs which may help pin down the root cause. Therefore the first bullet will help in diagnosis should this occur, the second step will ensure user data integrity, and the third step is to prevent the Supervisor from doing anything wacky.

Further investigation is required to nail down the cause of this issue.