devmapper: Add --opt options for devicemapper settings

This allows setting these settings to be passed:
dm.basesize
dm.loopbackdatasize
dm.loopbackmetadatasize

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)