Add --log-level support

Next steps, in another PR, would be:
- make all logging go through the logrus stuff
- I'd like to see if we can remove the env var stuff (like DEBUG) but we'll see

Closes #5198

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/sources/reference/commandline/cli.md

@@ -75,6 +75,8 @@ expect an integer, and they can only be specified once.
       --ip-forward=true                          Enable net.ipv4.ip_forward
       --ip-masq=true                             Enable IP masquerading for bridge's IP range
       --iptables=true                            Enable Docker's addition of iptables rules
+       -l, --log-level="info"                    Set the logging level
+
       --mtu=0                                    Set the containers network MTU
                                                    if no value is provided: default to the default route MTU or 1500 if no default route is available
       -p, --pidfile="/var/run/docker.pid"        Path to use for daemon PID file