rgw/s3: apply vhost logic to s3 only

rgw/s3: apply vhost logic to s3 only

the vhost-style transformations ran in RGWREST::preprocess() before we
even route the request, so applied to every REST API in radosgw

vhost-style requests are specific to the S3 API, so they should only
apply after being routed to RGWRESTMgr_S3

extract the vhost logic from RGWREST::proprocess() into
rgw_rest_transform_s3_vhost_style(), and call that only from
RGWRESTMgr_S3::get_resource_mgr_as_default()

url-decoding of request_uri into decoded_uri is now duplicated in
preprocess() to apply to all requests, then again after vhost-style
transforms the request_uri

Signed-off-by: Casey Bodley <cbodley@redhat.com>