devmapper: Add prefix to options in README.md

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: README.md remove leftover text

There was a "and is created with a fixed" chunk of text that was
a copy-paste leftover. Removed it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devicemapper: Probe what filesystem to use when mounting

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Allow specifying filesystem for thin devices

This adds the following --opts for the daemon:
dm.fs: The filesystem to use for the base image
dm.mkfsarg: Add an argument to the mkfs command for the base image
dm.mountopt: Add a mount option for devicemapper mount

Currently supported filesystems are xfs and ext4.
devmapper: Add options for specifying block devices

This adds dm.datadev and  dm.metadatadev options that you can use
with -o to set to specific devices to use for the thin provisioning
pool.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Add blkdiscard option and disable it on raw devices

The blkdiscard hack we do on container/image delete is pretty slow, but
required to restore space to the "host" root filesystem. However, it
is pretty useless on raw devices, and you may not need it in development
either.

In a simple test of the devicemapper backend on loopback the time to delete
20 container went from 11 seconds to 0.4 seconds with -o blkdiscard=false.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)