rgw/sal: move User::create_bucket() to Bucket::create()

rgw/sal: move User::create_bucket() to Bucket::create()

the CreateBucket operation always has to deal with existing buckets, so
we have to load the Bucket handle first anyway

also moves the related placement and forward_request_to_master() logic
out of sal entirely

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