Persistent file system user ID enforcement

[jameshcorbett]

On hetchy, I created a persistentstorageinstance named "corbett".

When @behlendorf attempted to create a persistent instance named "corbett", he ran into the following error:

[behlendo@hetchy12:src]$ flux run --setattr=system.dw="#DW create_persistent capacity=10GiB type=lustre name=corbett" -N1 -n1 -c1 hostname
17.640s: job.exception type=exception severity=0 DWS/Rabbit interactions failed: workflow in 'Error' state too long: DW Directive 0: User ID does not match existing persistent storage

Which is fine, although the error message could be improved with #46.

However, @behlendorf was able to use my persistent storage instance:

[behlendo@hetchy13:src]$ ls -l /mnt/nnf/a8bf5c2e-f54d-46a3-8f6d-e2372b347c9c-0/
total 25
-rw-r--r-- 1 corbett8 corbett8  5 Jun 28 16:00 hello2.txt
-rw-r--r-- 1 corbett8 corbett8 12 Jun 28 15:59 hello.txt

This has the advantage of allowing groups to share persistent storage, by creating a single instance with the right group permissions set. But I'm wondering if this is the right way to enable that.