Deploy locks

Add a deploy lock for commands that are unsafe to run concurrently.

The lock is taken by creating a `mrsk_lock` directory on the primary
host. Details of who took the lock are added to a details file in that
directory.

Additional CLI commands have been added to manual release and acquire
the lock and to check its status.

```
Commands:
  mrsk lock acquire -m, --message=MESSAGE  # Acquire the deploy lock
  mrsk lock help [COMMAND]                 # Describe subcommands or one specific subcommand
  mrsk lock release                        # Release the deploy lock
  mrsk lock status                         # Report lock status

Options:
  -v, [--verbose], [--no-verbose]                # Detailed logging
  -q, [--quiet], [--no-quiet]                    # Minimal logging
      [--version=VERSION]                        # Run commands against a specific app version
  -p, [--primary], [--no-primary]                # Run commands only on primary host instead of all
  -h, [--hosts=HOSTS]                            # Run commands on these hosts instead of all (separate by comma)
  -r, [--roles=ROLES]                            # Run commands on these roles instead of all (separate by comma)
  -c, [--config-file=CONFIG_FILE]                # Path to config file
                                                 # Default: config/deploy.yml
  -d, [--destination=DESTINATION]                # Specify destination to be used for config file (staging -> deploy.staging.yml)
  -B, [--skip-broadcast], [--no-skip-broadcast]  # Skip audit broadcasts
```

If we add support for running multiple deployments on a single server
we'll need to extend the locking to lock per deployment.

test/commands/lock_test.rb

@@ -0,0 +1,33 @@
+require "test_helper"
+
+class CommandsLockTest < ActiveSupport::TestCase
+  setup do
+    @config = {
+      service: "app", image: "dhh/app", registry: { "username" => "dhh", "password" => "secret" }, servers: [ "1.1.1.1" ],
+      traefik: { "args" => { "accesslog.format" => "json", "metrics.prometheus.buckets" => "0.1,0.3,1.2,5.0" } }
+    }
+  end
+
+  test "status" do
+    assert_equal \
+      "stat mrsk_lock > /dev/null && cat mrsk_lock/details | base64 -d",
+      new_command.status.join(" ")
+  end
+
+  test "acquire" do
+    assert_match \
+      /mkdir mrsk_lock && echo ".*" > mrsk_lock\/details/m,
+      new_command.acquire("Hello", "123").join(" ")
+  end
+
+  test "release" do
+    assert_match \
+      "rm mrsk_lock/details && rm -r mrsk_lock",
+      new_command.release.join(" ")
+  end
+
+  private
+    def new_command
+      Mrsk::Commands::Lock.new(Mrsk::Configuration.new(@config, version: "123"))
+    end
+end