rgw: make radosgw-admin user create and modify distinct

rgw: make radosgw-admin user create and modify distinct

Currently if you run 'radosgw-admin user create ..' when the user
already exists and you happen to specify, at least,  '--uid' and
'--display-name' that match the existing user, radowgw-admin will
actaully go modify the existing user.

This behaviour is a little confusing, hence the bug this patch is
fixing. This patch instead simplifies the tool to make
'create' create and 'modify' modify.
Meaning when you go 'create' a user that already exists, you'll get an
error, as expected. If you want to modify a user, you actually have to
use 'modify'.

For exapmle, now:

  $ radosgw-admin user create --uid="test-user"  --display-name="test user"
  could not create user: unable to create user, user: test-user exists

Signed-off-by: Matthew Oliver <moliver@suse.com>
Fixes: https://tracker.ceph.com/issues/38619