devmapper: Add --storage-opt options for basic devicemapper settings

This allows setting these settings to be passed:
dm.basesize
dm.loopdatasize
dm.loopmetadatasize

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

daemon/graphdriver/devmapper/README.md

@@ -0,0 +1,73 @@
+## devicemapper - a storage backend based on Device Mapper
+
+### Theory of operation
+
+The device mapper graphdriver uses the device mapper thin provisioning
+module (dm-thinp) to implement CoW snapshots. For each devicemapper
+graph locaion (typically `/var/lib/docker/devicemapper`, $graph below)
+a thin pool is created based on two block devices, one for data and
+one for metadata.  By default these block devices are created
+automatically by using loopback mounts of automatically creates sparse
+files.
+
+The default loopback files used are `$graph/devicemapper/data` and
+`$graph/devicemapper/metadata`. Additional metadata required to map
+from docker entities to the corresponding devicemapper volumes is
+stored in the `$graph/devicemapper/json` file (encoded as Json).
+
+In order to support multiple devicemapper graphs on a system the thin
+pool will be named something like: `docker-0:33-19478248-pool`, where
+the `0:30` part is the minor/major device nr and `19478248` is the
+inode number of the $graph directory.
+
+On the thin pool docker automatically creates a base thin device,
+called something like `docker-0:33-19478248-base` of a fixed
+size. This is automatically formated on creation and contains just an
+empty filesystem. This device is the base of all docker images and
+containers. All base images are snapshots of this device and those
+images are then in turn used as snapshots for other images and
+eventually containers.
+
+### options
+
+The devicemapper backend supports some options that you can specify
+when starting the docker daemon using the --storage-opt flags.
+This uses the `dm` prefix and would be used somthing like `docker -d --storage-opt dm.foo=bar`.
+
+Here is the list of supported options:
+
+ *  `dm.basesize`
+
+    Specifies the size to use when creating the base device, which
+    limits the size of images and containers. The default value is
+    10G. Note, thin devices are inherently "sparse", so a 10G device
+    which is mostly empty doesn't use 10 GB of space on the
+    pool. However, the filesystem will use more space for the empty
+    case the larger the device is.
+
+    Example use:
+
+    ``docker -d --storage-opt dm.basesize=20G``
+
+ *  `dm.loopdatasize`
+
+    Specifies the size to use when creating the loopback file for the
+    "data" device which is used for the thin pool. The default size is
+    100G. Note that the file is sparse, so it will not initially take
+    up this much space.
+
+    Example use:
+
+    ``docker -d --storage-opt dm.loopdatasize=200G``
+
+ *  `dm.loopmetadatasize`
+
+    Specifies the size to use when creating the loopback file for the
+    "metadadata" device which is used for the thin pool. The default size is
+    2G. Note that the file is sparse, so it will not initially take
+    up this much space.
+
+    Example use:
+
+    ``docker -d --storage-opt dm.loopmetadatasize=4G``
+