Add migration between persistent_homes true/false

[ionphractal]

This PR makes it possible to smoothly transition between persistent_homes true/false without loosing data.

The following premises apply for this:

• if you migrate from false (volatile homes), the "bosh deploy" step does not recreate the VMs
• no user is logged or has files open in during the time of the upgrade (open work will be lost because data is moved accross devices)
• the respective destination folder (new_home_dir/user) is not already present
• the destination disk has enough free disk space

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/171493221

The labels on this github issue will be updated when the story is started.

[h4xnoodle]

Hi @ionphractal can you describe some of the testing you've done to ensure this works and is compatible with current stemcells? Is this something specific for your environment or would benefit any consumer of this release?

[ionphractal]

@h4xnoodle Sorry for the late reply. This wasn't on my top high prio list and at some point fell under the desk.

I "tested" with a rather old deployment we have with multiple users on it that grew over time in terms of users. Not sure how some users ended up having a different naming structure but I think it was because we had volatile homes in the beginning, created users and switched to persistent homes. But at some point we decided to remove persistent homes again and the deployment update failed because a user folder wasn't where it was expected to be. So I made a dev release with the changes and from this PR on successfully was able to switch now back and forth between volatile and persistent homes.

[bosh-admin-bot]

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[bosh-admin-bot]

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