Update vstart to setup users for s3-tests

vstart.sh now creates the users for the default configuration
for the s3-tests, available on https://github.com/ceph/s3-tests.

Also updated the documentation to show the correct RadosGW port.

Signed-off-by: Luis Pabón <lpabon@redhat.com>
Merge pull request #2712 from lpabon/bug-8766

Update vstart to setup users for s3-tests