graphdriver: build tags

Enable build tags for all the graphdrivers to be excludable.

As an example:
```
$ go build
$ ls -l docker
-rwxr-xr-x 1 vbatts vbatts 18400158 Mar 14 14:22 docker*
$ go build -tags "exclude_graphdriver_aufs exclude_graphdriver_vfs exclude_graphdriver_devicemapper"
$ ls -l docker
-rwxr-xr-x 1 vbatts vbatts 17467068 Mar 14 14:22 docker*
```

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
runtime: no build tags for vfs driver

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
add env hint

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
devmapper: Increase timeout in waitClose to 10sec

As reported in https://github.com/dotcloud/docker/issues/4389 we're
currently seeing timeouts in waitClose on some systems. We already
bumped the timeout in waitRemove() in
https://github.com/dotcloud/docker/issues/4504.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4738 from SvenDowideit/mention-env-scrubbing

add env hint
Merge pull request #4739 from alexlarsson/dm-timeout

devmapper: Increase timeout in waitClose to 10sec
Merge pull request #4674 from vbatts/vbatts-graphdriver_build_tags

graphdriver: build tags