rgw: use correct default rgw frontend address

rgw: use correct default rgw frontend address

since 0.0.0.0 is the default radosgw address (not 'address'), not
configuring an address explicitly, and instead configuring the radosgw
interface, would result in 0.0.0.0 being used, instead of falling
through to section that inspects the interface config option.

backport note: this cannot be cherry-picked from master since this code
doesn't exist in master.

fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1655131

Signed-off-by: Noah Watkins <nwatkins@redhat.com>