Remove --read-only restriction when user ns enabled

The restriction is no longer necessary given changes at the runc layer
related to mount options of the rootfs. Also cleaned up the docs on
restrictions left for userns enabled mode. Re-enabled tests related to
--read-only when testing a userns-enabled daemon in integration-cli.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

docs/reference/commandline/dockerd.md

@@ -955,16 +955,16 @@ This option will completely disable user namespace mapping for the container's u
 The following standard Docker features are currently incompatible when
 running a Docker daemon with user namespaces enabled:
 
- - sharing PID or NET namespaces with the host (`--pid=host` or `--network=host`)
- - A `--read-only` container filesystem (this is a Linux kernel restriction against remounting with modified flags of a currently mounted filesystem when inside a user namespace)
- - external (volume or graph) drivers which are unaware/incapable of using daemon user mappings
+ - sharing PID or NET namespaces with the host (`--pid=host` or `--net=host`)
  - Using `--privileged` mode flag on `docker run` (unless also specifying `--userns=host`)
 
 In general, user namespaces are an advanced feature and will require
 coordination with other capabilities. For example, if volumes are mounted from
 the host, file ownership will have to be pre-arranged if the user or
 administrator wishes the containers to have expected access to the volume
-contents.
+contents. Note that when using external volume or graph driver plugins, those
+external software programs must be made aware of user and group mapping ranges
+if they are to work seamlessly with user namespace support.
 
 Finally, while the `root` user inside a user namespaced container process has
 many of the expected admin privileges that go along with being the superuser, the