Fix s3 tests in the rgw workunit.

Make it possible to set RGW_PORT with ENV variable.

Fixes: 7500
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #1736 from ceph/wip-7500-wusui

Fix s3 tests in the rgw workunit.

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>