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.

lib/mrsk/commands/lock.rb

@@ -0,0 +1,63 @@
+require "active_support/duration"
+require "active_support/core_ext/numeric/time"
+
+class Mrsk::Commands::Lock < Mrsk::Commands::Base
+  def acquire(message, version)
+    combine \
+      [:mkdir, lock_dir],
+      write_lock_details(message, version)
+  end
+
+  def release
+    combine \
+      [:rm, lock_details_file],
+      [:rm, "-r", lock_dir]
+  end
+
+  def status
+    combine \
+      stat_lock_dir,
+      read_lock_details
+  end
+
+  private
+    def write_lock_details(message, version)
+      write \
+        [:echo, "\"#{Base64.encode64(lock_details(message, version))}\""],
+        lock_details_file
+    end
+
+    def read_lock_details
+      pipe \
+        [:cat, lock_details_file],
+        [:base64, "-d"]
+    end
+
+    def stat_lock_dir
+      write \
+        [:stat, lock_dir],
+        "/dev/null"
+    end
+
+    def lock_dir
+      :mrsk_lock
+    end
+
+    def lock_details_file
+      [lock_dir, :details].join("/")
+    end
+
+    def lock_details(message, version)
+      <<~DETAILS.strip
+        Locked by: #{locked_by} at #{Time.now.gmtime}
+        Version: #{version}
+        Message: #{message}
+      DETAILS
+    end
+
+    def locked_by
+      `git config user.name`.strip
+    rescue Errno::ENOENT
+      "Unknown"
+    end
+end