rgw: add support for multiple rgw instances on a single host

rgw: add support for multiple rgw instances on a single host

With this, we could have multiple rgw instances on a single host
with a single run, don't have to use rgw-standalone.yml which does not
seems able to bind ports separately.
If you want to have multiple rgw instances, just change 'radosgw_instances'
to the number you want, which defaults to 1.
Not compatible with Multi-Site yet.

Signed-off-by: guihecheng <guihecheng@cmiot.chinamobile.com>