vstart: start rgw on port specified by env var

This ensures that user specified option according to $CEPH_RGW_PORT is
considered before starting radosgw

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@ril.com>
Merge pull request #2327 from theanalyst/fix/vstart_rgw_port

vstart: start rgw on port specified by env var

Reviewed-by: Sage Weil <sage@redhat.com>