Deprecate "devicemapper" storage driver, and add warning

[thaJeztah]

Related deprecation note PR: docker/cli#1424

The devicemapper storage driver is deprecated in favor of overlay2, and will
be removed in a future release. Users of the devicemapper storage driver are
recommended to migrate to a different storage driver, such as overlay2, which
is now the default storage driver.

The devicemapper storage driver facilitates running Docker on older (3.x) kernels
that have no support for other storage drivers (such as overlay2, or AUFS).

Now that support for overlay2 is added to all supported distros (as they are
either on kernel 4.x, or have support for multiple lowerdirs backported), there
is no reason to continue maintenance of the devicemapper storage driver.

[thaJeztah]

ping @mavenugo @andrewhsu @AkihiroSuda

[thaJeztah]

/cc @rhvgoyal

[vdemeester]

LGTM 🐸

[thaJeztah]

d'oh! I moved the warning in the logs to the wrong location; let me update

[codecov]

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[thaJeztah]

Fixed; should now also warn when a driver is manually selected

[yongtang]

LGTM

[thaJeztah]

s390 failure is unrelated;

15:01:00 FAIL: docker_api_swarm_node_test.go:75: DockerSwarmSuite.TestAPISwarmNodeDrainPause
15:01:00 
15:01:00 [dff43245afe45] waiting for daemon to start
15:01:00 [dff43245afe45] daemon started
15:01:00 
15:01:00 [d3fcbc0359b0a] waiting for daemon to start
15:01:00 [d3fcbc0359b0a] daemon started
15:01:00 
15:01:00 docker_api_swarm_node_test.go:104:
15:01:00     waitAndAssert(c, defaultReconciliationTimeout*2, reducedCheck(sumAsIntegers, d1.CheckActiveContainerCount, d2.CheckActiveContainerCount), checker.Equals, instances)
15:01:00 docker_utils_test.go:435:
15:01:00     c.Assert(v, checker, args...)
15:01:00 ... obtained int = 3
15:01:00 ... expected int = 1
15:01:00 ... output: "f3bf56f27764\n81d3d1c9e4c6\n2bd7ac7e950c\n"
15:01:00 
15:01:00 [dff43245afe45] exiting daemon
15:01:00 [d3fcbc0359b0a] exiting daemon
15:01:36