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/cli/cli_test_case.rb

@@ -8,13 +8,14 @@ class CliTestCase < ActiveSupport::TestCase
     ENV["VERSION"]             = "999"
     ENV["RAILS_MASTER_KEY"]    = "123"
     ENV["MYSQL_ROOT_PASSWORD"] = "secret123"
+    Object.send(:remove_const, :MRSK)
+    Object.const_set(:MRSK, Mrsk::Commander.new)
   end
 
   teardown do
     ENV.delete("RAILS_MASTER_KEY")
     ENV.delete("MYSQL_ROOT_PASSWORD")
     ENV.delete("VERSION")
-    MRSK.reset
   end
 
   private