Support names for enforce_filesystem

[mcpherrinm]

Right now if you want to enforce tmpfs, you have to write something like:

filesystem_type: 16914836

That's weird and hard to read. We should have a map of string names.

[mcpherrinm]

Also we should have better error reporting when a filesystem is unexpected, to aid in debugging.

[mcpherrinm]

Also we should check before starting the sync, not that it really matters, but it'll make it fail slightly faster & without unneeded network traffi.