Increase to 255 the length of the user name.
Increase to 255 the length of the user name.

This change is needed by [1]. This won't change the results of the test
right now, since keystone rejects usernames longer than 64 and 255 > 64.

[1] https://review.openstack.org/#/c/22694/

Change-Id: Iaf81b223cd8f0ec41b9836e26a4c294c10d56893

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