10557: Tidy up users#setup controller.

10557: Tidy up users#setup controller.

Simplify long conditional, and fix bug where admin asks for repo
"username/reponame" but "username/username/reponame" gets created.

This also fixes an unpredictable API: Previously, if params included
{user:{uuid:X,email:Y}}, the setup API would either create a new user
with uuid X and email Y, or set up an existing user (ignoring Y),
depending on whether X was the UUID of an existing user. Now, passing
a "user" hash like this always tries to create a new user with
uuid=X (if given) and email=Y, and returns an error if the given UUID
is already in use.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curoverse.com>