Don't mount user home with alternate keys root

[PVince81]

Description

When encryption keys are stored outside the user's homes, there is no
need to mount said homes.

Related Issue

Follow up of #26917.
Discovered while working on #26935.
This only fixes the "alternative key storage" case from #26935 and might also improve the performance a bit as we don't need to mount the user's homes.

Motivation and Context

Don't do unnecessary stuff.
But also I feel that this was missing from #26917

How Has This Been Tested?

• unit tests
• use steps from Alternative key storage location doesn't delete all keys after deleting user #26936 on this PR and see that there is NoUserException when deleting a user in this "alternate keys storage root" setup.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Please review @jvillafanez @SergioBertolinSG

[mention-bot]

@PVince81, thanks for your PR! By analyzing the history of the files in this pull request, we identified @DeepDiver1975 to be a potential reviewer.

[jvillafanez]

Hmmm, does the alternate root refer to a full path on the host? If so, it should be fine, although I'd like another kind of flag for this.

[PVince81]

@jvillafanez the logic is as follows: empty string path means "no alternative root".
When a path is specified (full path on host, or relative to data dir (I forgot which)), then it uses that one.

Indeed a flag would be better here to make it clearer.

[lock]

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