btrfs: build tags

Default to the same build behavior, but allow a go build tag to disable
building of the btrfs graphdriver

```
$ go build
$ objdump -S docker | grep btrfs | wc -l
194
$ go build -tags no_btrfs
$ objdump -S docker | grep btrfs | wc -l
1
```
that is a comment ;-)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)